Hundreds of millimetres of rain is set to lash Sydney and the surrounding regions, with New South Wales potentially facing the most torrential downpour since the late 1990s.
Flash flooding, monster waves and high winds are expected to batter NSW over the weekend, as emergency services brace for the worst.
The Bureau of Meteorology said about 100mm was expected across parts of the state on Saturday - and double that could fall on Sunday - because of an east coast low.
"The rainfall will be variable but hundreds of millimetres. It's not something we see that often," BOM acting NSW state manager Jane Golding told reporters on Saturday.
"The multi-day rainfall from this event looks like much more than what we had during the June 2016 east coast low which was our last significant east coast low.
"Potentially, we haven't seen anything like this since the late 1990s."
BOM senior forecaster Jordan Notara on Saturday afternoon said they were seeing an intensification of a coastal trough, with very heavy rainfall over metropolitan Sydney and surrounds.
Soggy streets greated commuters as they travelled through inner-Sydney. Source: AAP
"We could see 100 to 200mm falling over the next 12 to 24 hours, which could see inundation on some places along the coast, minor to major flooding over some catchments and very strong winds along the coastline," he said.
The NSW SES said they had already rescued 13 carloads of people after rain battered the Central Coast on Friday night.
Mt Elliot (184mm), Toukley (159mm), Kangy Angy (140mm) and Wyong (125mm) received heavy falls from 9pm on Friday to 4am on Saturday.
A video was posted online of a man driving his jetski past a McDonald's down a flooded street in Tuggerah on the Central Coast.
It drew swift condemnation from NSW Police minister David Elliott, who labelled the driver a "boofhead" and asked police to investigate.
"We have emergency services deployed during a difficult job under difficult circumstances, and for them to have to divert their resources. It's just not good enough," Mr Elliott said.
The SES has over 600 volunteers on the ground as it prepares for the flash flooding on Saturday, with a severe weather warning current along the entire coast from the Northern Rivers Region to the South Coast.
Water overflows the banks of the Parramatta River after heavy rains buffeted Sydney. Source: AAP
The BOM is warning of potential flooding for the coast from the Tweed to Bega rivers, and says moderate-to-major flooding is possible in the Hawkesbury Nepean catchment, the Hunter Region and parts of the South Coast.
Strong winds were also forecast, with waves of between five-to-six metres set to batter the coast between Batemans Bay and the Central Coast.
With the fire-affected South Coast set to be lashed by heavy rain from late Saturday, the BOM warned that the burned-out landscape and loss of vegetation had left it vulnerable to landslips.
There were 40 fires still burning throughout the state on Saturday evening.
