Hundreds more people were evacuated Wednesday in Australia's flood-hit southeast, as residents of the city of Wagga Wagga breathed a sigh of relief after a levee on the Murrumbidgee River held firm.

About 600 people were ordered to leave their homes in Griffith, northwest of sodden Wagga Wagga, as floodwaters threatened the low-lying New South Wales town of 16,000 people.

State Emergency Services spokesman James McTavish told ABC radio it could become isolated.

"There is a very significant flood emergency there. There is the potential for that flood emergency in Griffith to continue for a couple of weeks and a large number of people have been evacuated in the last 24 hours," he said.

"We're hopeful though that the preparations that we're putting in place, today in particular, will protect more of Griffith and surrounds."

The evacuation order came as a severe weather warning was issued for Sydney, the south coast, and the Illawarra and Hunter regions of New South Wales with heavy rain forecast that could lead to flash flooding.

Floods have hit three eastern states this week, sweeping two men to their deaths after they attempted to cross waterways in cars, inundating hundreds of homes and causing millions of dollars in damage.

In Wagga Wagga, New South Wales's largest inland city, 9,000 people were evacuated on Tuesday as the surging Murrumbidgee threatened to breach its levee.

While around 240 homes in the north of the town were damaged -- some with water up to the roof -- many more in the central business district were spared after the river peaked just below the levee limit.

Some residents were given the all-clear to return home Wednesday, although parts of the town remained under water.

Red Cross national emergency services manager Andrew Coghlan warned flood victims to brace themselves for a long road ahead.

"People need to prepare themselves for the challenge of not only cleaning up and repairing damage to their property but also overcoming the disruption caused to family life and the community," he said.

New South Wales Premier Barry O'Farrell said it would be weeks or months before the real cost of the crisis was known while Prime Minister Julia Gillard warned of a "long, hard journey of recovery".

"As a federal government we'll keep working with our state counterparts and local communities to support them during this difficult time," said Gillard, who travelled to Wagga Wagga Wednesday to see the destruction for herself.

"As a nation we've shown that we don't leave people behind, that we continue to show our generosity and support to our fellow Australians when they're in times of need."

Around New South Wales more than 13,000 people have been asked to leave their homes due to flooding, with hundreds of properties inundated and a number of rural communities isolated by the rising waters.

Flooding has also hit rural regions in the state of Victoria with nervous residents of the town of Nathalia hoping an emergency levee built to protect 170 homes will hold as a creek keeps rising.

Peter Ryan, the deputy premier of Victoria, said he was confident it would.

"We will save this town," he told reporters.