Flooding, road closures and rising rivers wreak havoc

Torrential rain and flash flooding is wreaking havoc across greater Sydney with some residents preparing to evacuate and others told to cancel plans to attend the city's Vivid light festival.

Flooding across the river network has prompted the SES to warn people living in parts of Cobbitty, close to the Nepean River and Chipping Norton to prepare to leave as floodwaters rise.

Meanwhile, heavy rain has flooded tracks, closed Sydney railway stations and blocked major roads.

Train services were thrown into disarray on Friday, as floodwaters halted services in the city loop, Blue Mountains, Mittagong, Cockle Creek and Sandbridge.

Commuters along the T8 airport line were forced to take buses for the majority of the day before the flooded train line reopened just after 2pm.

Passengers trying to travel by air or water did not fare much better with several ferry services cancelled or stopping early and Sydney Airport briefly closed two runways.

The widespread transport mess prompted Transport for NSW's Howard Collins to tell people to consider not attending the opening night of the Vivid Sydney light festival on Friday.

Authorities are also keeping a close eye on the Hawkesbury-Nepean and Warragamba rivers with minor flooding in the Hawkesbury and Colo river systems expected.

Sydney Water has warned of likely overflow at Warragamba Dam, which was at 96 per cent capacity by 3pm on Friday.

More than 200mm of rain fell in Richmond, about 90mm in Camden, more than 50mm in Katoomba and at least 75mm at Sydney Airport in the 24 hours to 9am on Friday.

Parts of the Illawarra copped more than 200mm, including Clover Hill, causing localised flooding on the coast.

But the trough, which also left 50,000 people isolated across the mid-north coast and Hunter regions earlier in the week, was beginning to lose intensity, the Bureau of Meteorology said.

"Whilst the rainfall is significant today, it won't match the 200-300mm we were getting earlier in the week," senior meteorologist Angus Hines said.

"We don't expect the rivers to peak too much higher than what they are at the moment ... hopefully starting to come down today or through the course of the weekend."