UPDATE: A volunteer helping the State Emergency Service during today's once-in-a-generation storm has died.
The NSW SES volunteer died while on duty in Sydney, according to early reports. In a statement, the NSW branch said the volunteer "passed away after collapsing while on duty".
The wild weather is believed to have contributed to a second death on Sydney roads when a 14-year-old old boy was killed following a two-vehicle crash in Thornleigh, on Sydney's upper north shore.
The Hornsby Advocate reports the Year 8 student was a passenger in a Toyota Corolla being driven by a 17-year-old boy. The car crashed into a Landrovver Discovery and the boy died at the scene.
Two police officers were also seriously injured when a tree fell on their car further south in North Ryde.
The rain that belted large parts of the state may have subsided slightly but forecasters have warned Sydneysiders not to become complacent.
The intense low pressure system that delivered Sydney's wildest November storm came with a "second phase" as ferocious 90km/h winds teamed up with driving rain to make the commute home a misery.
"The band has drifted south but it will then come north again and as the low intensifies it will bring powerful winds and further rainfall. There will be plenty of trees coming down at the back end of the day," Sky News Weather channel meteorologist Rob Sharpe told news.com.au.
The Bureau of Meteorology warned shortly before 5pm that a "second phase is now beginning", bringing with it "storms and intense rain".
EARLIER: MORE than one months worth of rain has fallen in two hours across Sydney this morning with authorities pleading with commuters to consider staying at home.
Between 5.30am and 7.30am 90mm of rain fell on the main Sydney weather centre - the average for November is 67mm. Half of that fell in just 30 minutes.
Sydney Airport is suffering from huge delays with just one runway open.
Airport spokeswoman Cait Tynan said passengers should check with their airlines before they come to the airport.
"The airlines are the ones that make decisions about delays," she said.
On terra firma, confused commuters say they are stuck on buses with roads flooded, walking into blacked out train stations and sitting behind the wheel as traffic backs up.
Authorities have issued a severe thunderstorm warning for Sydney with wild weather already producing heavy rainfall and flash flooding.
It is so bad that Sydneysiders are being urged to reconsider their morning commutes. Police have said to stay off the roads and "work from home" amid warnings to expect months-worth of rain in just a few hours.
Visibility on many Sydney roads was greatly reduced as motorists faced long delays, and there are chaotic scenes at train stations across the city.
It is equally bad at Sydney's airport, with airlines cancelling multiple flights.
Airlines said travellers from Sydney, Brisbane and the Gold Coast should prepare for significant delays with adverse conditions making flying dangerous and difficult.
One passenger said there was chaos with "everything on lockdown, no flights in or out".
At Sydney airport, which is closed due to the storm, and they just started canceling flights. I counted five, but not mine... yet. Is dirty out on the roads. Stay home if you can!!! #sydneyairport #Sydneystorm— elise davidson (@elisedavidson) November 27, 2018
Forecasters had earlier warned heavily populated areas of New South Wales were set for a soaking today, with wild weather and torrential downpours predicted across the state.
Up to 100mm was expected to fall across Sydney, the Hunter and Illawarra, the Bureau of Meteorology said - well above the monthly average.
Some places could top 200mm as an intense low pressure area zeros in on the cities.
#sydneystorm currently demonstrating that previously I have over-used the phrase “tipping it down”.— jonathan jb webb (@jjbw) November 27, 2018
Here’s what it really means:
**actual bucketfuls of water landing around you without gaps in them** pic.twitter.com/h2RbFx3KX5
Sydney rivers including the Hawkesbury, Nepean, Cooks and Georges are all on flood watch, while the State Emergency Service is preparing to mobilise thousands of volunteers.
It's unclear where the rainfall will be most intense, however the weather bureau has warned of damaging winds and possible flash flooding, expected to reach the NSW coast during a morning peak lasting for six to 12 hours.
"We're asking all road users to perhaps reconsider the need to be on the road through what will be a severe rain event," NSW Police Chief Inspector Phillip Brooks told reporters on Tuesday.
"Drivers, riders, cyclists and pedestrians need to make sure they are safe on our roads."
⚠️ #Flood Watch issued for #HawkesburyNepean, #GeorgesRiver and #CooksRiver. Minor to moderate flooding from Wednesday. Very heavy rain forecast. See https://t.co/uJHnKkADTA for details and updates; follow advice from @NSWSES. #NSWFloods pic.twitter.com/DJ4IywQ0YK— Bureau of Meteorology, New South Wales (@BOM_NSW) November 27, 2018
NSW State Emergency Service Assistant Commissioner Scott Hanckel said parents should think about alternatives for the school drop-off and pick-up and advised businesses to expect workers to arrive late.
"It's a great day to work from home - if that's suitable," he told AAP. He warned drivers not drive into flood waters, with that being the overwhelming major cause of flood deaths.
Sheep graziers were warned cold temperatures, rain and strong south to south-easterly winds were expected, increasing the risk of losses to lambs and sheep exposed to these conditions.
Areas likely to be affected include the Illawarra, Southern Tablelands and Australian Capital Territory forecast districts and parts of the South Coast, Central Tablelands, South West Slopes and Snowy Mountains forecast districts.
Simon Lewis, New South Wales Severe Weather manager for the Bureau of Meteorology, said the downpour could lead to roads awash with water, rapidly rising creeks and hazardous surf until Friday morning.
However, he said the "one good thing" about the system was that it would pass quickly.
"It will cross the coast somewhere between Sydney and Wollongong tomorrow and then move fairly rapidly offshore," Mr Lewis predicted.
"We're not expecting a very long duration of heavy rain, but we are expecting to see quite intense falls sometime in the morning and persist through until the afternoon."
The culprit is a large low pressure system developing over the state, expected to rapidly intensify as it crosses the coast between Sydney and Wollongong early this morning.
EARLIER: We knew it was going to rain in NSW midweek. Just not by this much.
Meteorologists have dramatically upped the rain forecast for New South Wales with warnings of possible flash flooding amid torrential downpours in an intense six to 12 hour burst on Wednesday.
Meanwhile, in Queensland "wild supercell thunderstorms" could hit the south east of the state on Wednesday while central areas continue to wither under a heatwave stoking bushfires.
"A dangerous rain and wind event is on the way for NSW with Sydney and Wollongong in the firing line with heavy rain, followed by powerful winds as a low pressure system comes in," said Sky News Weather channel meteorologist Rob Sharpe.
"It's going to be a vigorous system - short and sharp".
It had been forecast that Sydney could see up to 50mm of rainfall over a two-day period beginning on Wednesday. Now the Bureau of Meteorology (BOM) is saying from around 60mm to as much as 120mm could bucket down on the city in just a few hours as a concentrated low pressure area zeros in.
To put that into perspective, November's average rainfall in Sydney is 66mm. So the city could see almost two months of rain in a single day - or even just a few hours.
FLOODING RAINS, DANGEROUS WINDS
And it's not just Sydney. Wollongong could see as much as 150mm of rain, Nowra 100mm, Katoomba 90mm and Wagga and Canberra 30mm. Some isolated coastal and exposed areas could be walloped with as much as 200mm.
"Current forecasts indicate intense rain could occur in major population centres from Wednesday morning. Damaging winds and hazardous surf conditions are also anticipated," said the BOM in an alert.
A severe weather warning is in place for the NSW coast from north of Newcastle to south of Nowra and inland to the Southern Highlands and Blue Mountains. But rain, although less of it, can be expected across much of the state.
The culprit is a low pressure system that is expected to enter the northwest of NSW on Tuesday and then track southeast, rapidly intensifying as it crosses the coast about the Greater Sydney area during Wednesday.
Mr Sharpe told news.com.au the severe weather warning for damaging winds was also worth noting.
"The most significant feature of the winds is that they peak after most of the rainfall has arrived. This means that the ground will be soggy and a little bit looser making it easier for trees to come down," he said.
"This event is likely to lead to flooding, trees and powerlines down and therefore we'll also have power outages with the worst of the weather."
Other capitals will be generally dry with just the odd spot of rain in Melbourne. But parts of Queensland could also get a battering.
'WILD' THUNDERSTORMS AND BUSHFIRES
Mr Sharpe said there was a chance of wild thunderstorms for parts of southeast Queensland on Wednesday.
"One or two supercell thunderstorms are possible near to the coast around lunchtime on Wednesday, most likely between Bundaberg and the Sunshine Coast, although they cannot be ruled out in Brisbane.
"Those that get hit could be hammered by destructive winds and giant hail."
Further north in Queensland, they would gladly take some on NSW's spare rain as the battle continues to dampen down a series of fierce bushfires.
The bushfire emergency in central Queensland could continue for days as fire fighters desperately try to control a huge fire that's forced hundreds of people from their homes.
A week of high temperatures is forecast for much of the state, leaving firefighters on edge as they try to protect communities near a vast bushfire that's already scorched 17,000 hectares.
Fire authorities ordered more people out of their homes on Monday, as a Boeing 737 water bomber was brought in from NSW.
The state government declared a disaster situation for communities that have already been hit by fire, or might be in the days ahead.
It means police can forcibly remove residents from the Baffle Creek Catchment, and from Wartburg, Deepwater, Agnes Water, Round Hill, Miriam Vale and Bororen if they refuse orders to leave.
They can also stop residents from returning home until it's safe. About 100 NSW fire fighters will join the battle on Tuesday, giving exhausted crews who've been on the fireground since Friday a chance to rest. Authorities fear the damage bill could soar, but only two homes are confirmed as being lost in the Deepwater area.
As hot, windy conditions roll on, Queensland Fire Commissioner Katarina Carroll warned the fire danger was far from over.
"The next seven days are extremely concerning for us," she told reporters on Monday.