The Bureau of Meteorology has issued flood watch alerts, with eyes on a deepening east coast low brooding off the New South Wales coast.
The State Emergency Service is on alert from the mid-north coast down to the Illawarra.
The SES is asking people to make sure storm drains are clear, and to park cars undercover if possible.
The Weather Bureau says a large low pressure system is due to develop over the next 24 hours, bring heavy rainfall and gale force winds.
There are flood watch alerts , with a greater than 70 per cent chance of flooding for the Nepean, Georges, Wyong and Paterson and Williams Rivers. A king tide could cause flooding in tidal zones of the Cooks and Woronora Rivers.
The rain is expected to increase from late Sunday.
SES spokeswoman Becky Gollings says now is the time to prepare.
"People living in rural areas and along rivers and streams should move any livestock and equipment up to higher ground before the wet weather sets in," she said.
"People (should) also avoid those low-lying areas that are prone to flashing flooding and rivers and creeks and storm water drains.
"That's where we see a lot of people get into trouble."
© ABC 2013
15:23 EST The weather pattern has altered abruptly, as it often does at this time of year, with a high pressure ridge taking over from low pressure, allowing almost all of New South Wales to dry out.