Sydney is experiencing a sodden start to autumn, following the wettest summer in the city in five years.
Parts of Sydney were drenched in nearly 50 millimetres of rain overnight, with the downpour expected to continue into the early afternoon.
Other parts of the state have been hit with more significant rainfall totals.
Orange Airport recorded 105 millimetres of rain overnight, almost as much as the area received throughout the entire summer, while Newcastle has been drenched in widespread rainfall totals of 60 millimetres.
Some domestic flights were delayed at Sydney Airport on Friday morning, while a section of Old South Head Road at Vaucluse collapsed, closing out-bound lanes to traffic.
But Weatherzone meteorologist Rob Sharpe said conditions were forecast to improve in Sydney on Saturday, when thousands of revellers will head into the city for the gay and lesbian mardi gras.
"It's looking like it's going to be a fairly cloudy day, and there's going to be a few showers coming through [on Saturday]," Mr Sharpe said.
"At the moment the rain is going to be off rather than on. Most of the day will be dry but there are going to be a few showers coming through."
Mr Sharpe said this summer was the wettest in five years, with 348.8 millimetres of rain falling - more than 50 millimetres above the summer average. Most of that rain fell in the last 33 days of summer.
But this summer was also notable in that Sydney experienced its hottest ever day on January 18, when the mercury topped 45.8 at Sydney's Observatory Hill. That broke the previous record set in 1939 by half a degree.
Sydney also recorded its driest December in eight years.
Mr Sharpe said a low-pressure trough was moving through NSW on Friday morning.
A high pressure system was also centred to the west of Tasmania, extending a ridge along the southern coast of New South Wales.
In Sydney on Friday, the rain is expected to ease in the early afternoon, however winds are expected to average 30-50km/h, with gusts of up to 70km/h.
A maximum of 21 degrees was forecast in the city on Friday, with a top of 23 degrees on Saturday and 25 on Sunday.
A Transport Management Centre spokesman said part of Old South Head Road collapsed at Vaucluse, near the intersection with Belah Avenue, early on Friday morning.
He said lanes heading away from the city were closed while emergency services and the council examined the site. Traffic is being diverted via Cambridge Avenue and Derby Street.
About 9000 households around Doonside were blacked out when a cable failed at the Doonside substation about 4.20am.
Residents in the area reported hearing a loud boom at the time. Endeavour Energy said most households were back online about 6.50am, with about 100 homes still affected.
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