A month of rain is forecast to fall in 48 hours, with flood watch alerts issued for South Coast rivers.

Tuesday and Wednesday are the at-risk days, with a low pressure system deepening and storm activity increasing, especially across southern parts of NSW.

In the Illawarra up to 80 millimetres of rain is possible on Tuesday, with a thunderstorm likely.

On Wednesday up to 20mm of rain and a thunderstorm are predicted.

Further south, localised heavy rainfall totals of more than 200 millimetres are a possibility, with the region around Moruya the main area of concern for SES crews.

The remainder of the south east may see widespread totals of 20 - 40mm.

The SES has personnel and equipment on standby in the south-eastern parts of the state where the weather is expected to impact the most.

SES Chief Superintendent Dallas Burnes said flash flooding was a risk and can happen anywhere there is localised heavy rainfall.

"If it's flooded, stop and find an alternative route. Flash flooding can be deeper than it appears and can contain sewage and poisons, as well as hidden debris," he said.

"If you live, work or regularly commute through a low-lying area, there may be no official warning for flash flooding. It is critical you assess the conditions and also check live traffic before travelling."

In the Illawarra, rain will continue across the week with up to 5mm possible on Thursday and Friday, and 10-20mm each day on the weekend.

If you are trapped by rising flash floodwater, seek the safest refuge position and call triple-0 if life threatening.

For non-life threatening help during floods and storms call the SES on 132 500.