Weather: Urgent flood warnings issued as storms and heavy rain batter Australia's coastline

A possibly severe thunderstorm looks set to dump even more rain on Sydney as a sustained wet period drags on.

Parts of the NSW south coast have been issued with severe weather warnings as 70 to 200mm of rain could fall.

The Bureau of Meteorology is forecasting certain rain in and around Sydney for Saturday, with the chance of a thunderstorm near the coast which could become severe.

Surf conditions will be severe, the bureau forecasts. Hazardous surf warnings are in place for the Macquarie, Hunter, Sydney and Illawarra coasts Saturday, and Hunter and Sydney for Sunday.

Earlier a bureau meteorologist said there was potential for severe thunderstorms about the Sydney metropolitan area and Illawarra, to bring heavy rainfall that may lead to flash flooding.

Anywhere from 20 to 45mm of rain is expected around Sydney on Saturday, with three to 20mm on the outlook for Sunday.

There was the potential for up to 200mm of rain within 24 hours in some southern parts of the state and a flood watch would be issued, the meteorologist said.

The bureau has issued warnings that heavy rainfall could lead to flash flooding in northern parts of the South Coast, southern parts of the Illawarra, eastern parts of the Southern Tablelands and far northeastern parts of Snowy Mountains districts from Saturday morning into Sunday.

'Six-hourly rainfall totals between 70 and 90mm, with isolated 100 to 120mm possible. 24 hourly totals between 80 to 120mm are likely and isolated falls up to 200mm are possible over the ranges,' the forecast reads.

Sydney has smashed its record rainfall for the start of May with nearly 150mm recorded in the Observatory Hill gauge since the start of the month.

The bureau reckons up to four millimetres could fall on Monday and then there's a 20 per cent chance of any rain Tuesday. If the rain clouds stay away that will snap Sydney's 13 day wet streak.

This weekend's storm is expected to bring to southern Queensland and northern Victoria.