The heaviest falls in Sydney are predicted to be on Tuesday and Wednesday with up to 70 and 90 millimetres on those days.

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On the South Coast, Kiama is forecast to get up to 80 millimetres on Tuesday and 150 millimetres on Wednesday, and Batemans Bay is bracing itself for similarly heavy falls on Wednesday and Thursday.

Kempsey mayor Leo Hauville said all the low-level bridges along the Macleay River were already covered by floodwater and his community was bracing themselves for rising waters.

“If the weather system continues to travel south it’ll get to us in 24 hours,” he said, noting farmers were moving stock to higher ground.

“The whole area is wary and watching and hoping for the best.”

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The bureau is predicting the Macleay River at Kempsey could reach minor flood level of 4.5 metres on Tuesday night.

The widespread nature of the floods up and down the coast will stretch the state's disaster resources. Commissioner York said her agency resisted sending more people up north to ensure there was capacity down south when needed.

"We have moved extra members up into the Northern Rivers district, but also we have retained our volunteers and other emergency service partner members down on the South Coast to make sure that they are in the stages of planning, and also getting messaging out and communication out to the community to prepare them for this event as it comes through," she said.

Commissioner York said, while Sydney could get heavy rain, it was unlikely to be as heavy as the storms up north, but could still lead to flash flooding.

"That is affected by the amount of saturation that's in the land at the moment that there is no way for the water to run as it falls onto the ground, which will cause flash flooding and that's a very dangerous situation for any motorists."

The SES reminds people in flood-affected areas:

• Never drive through flood waters.
• Clean out your gutters.
• Tie down loose equipment.
• Prepare an emergency pack in case you have to leave. Include food, water, medication, important documents, a torch, battery pack and a radio.