Heavy rainfall up to 250mm about to hit Southern Inlands

"A surface trough extending from the Illawarra region into the Tasman Sea will deepen during Thursday in response to an upper low over NSW," a BoM spokesperson said.

"A strengthening humid southeasterly flow to the south of this trough will lead to persistent rain about the Illawarra, northern parts of the South Coast and adjacent Southern and Central Tablelands during Thursday and into Friday."

Up to 250mm is possible over a 24-hour period and residents are being warned of flash flooding.

"Heavy falls are more likely in southern parts of the warning area from early morning, before spreading to northern parts of the area in the afternoon and evening," the spokesperson said.

"Six-hourly rainfall totals between 50 and 90 millimetres are likely with isolated totals of 120 millimetres possible on the windward ranges.

"Rainfall totals between 100 and 150 millimetres are likely with isolated totals of 250 millimetres over 24-hours."

Conditions are expected to ease during Friday morning as the trough moves south, however there is some uncertainty in the longer term movement and development of the system.

Information on locations including Bowral, Taralga and Nerriga is available online.

The State Emergency Service (SES) has issued a warning for residents to not drive, ride or walk through flood water, and to keep clear of creeks and storm drains.

"If you are trapped by flash flooding, seek refuge in the highest available place and ring 000 if you need rescue," a spokesperson said.

"Be aware that runoff from rainfall in fire affected areas may behave differently and be more rapid. It may also contain debris such as ash, soil, trees and rocks."

The SES is available for emergency help in floods and storms by phoning 132 500.