SYDNEY - Evacuation orders covering more than 30,000 people have been imposed in flood-ravaged areas of Sydney as residents in some parts of the city faced their third major flooding disaster this year.

As the Australian military was deployed to assist with evacuations, emergency services on Monday (July 4) warned residents in affected areas across Sydney to stay off the roads and to be ready to evacuate if ordered.

The floods followed days of relentless rain that caused rivers and dams to overflow and left houses and businesses underwater.

For some residents in Sydney, the disaster marked the repeat of similar flooding events experienced as recently as March and April.

In the south-west suburb of Camden, a resident, Ms Hayley Linea, said her family had already endured three floods since moving there earlier this year.

"We've only lived here for six months and out of the three big floods we've had, this is the worst," she told the Illawarra Mercury newspaper.

"I think it's going to be a bit of a journey but all things considered, we're safe… We just didn't know (the flood water) was going to come up that far, that quick."

Some parts of Sydney are facing serious flooding for the fourth time in 18 months, though this week's disaster was set to be the city's worst. Major flooding across the east coast of Australia in March killed at least 20 people.

Following the recent rain in and around Sydney, Warragamba Dam, the city's main water reservoir, began spilling on Sunday morning.

The dam in the outer south-west has been sending huge volumes of water into nearby rivers, which have already overflowed or are approaching flood levels.

The flooding across Sydney has left bridges submerged and disrupted roads and public transport services.

The federal government deployed 100 Australian Defence Force personnel and two helicopters to assist with searches and evacuations.

New South Wales (NSW) Premier Dominic Perrottet on Monday requested that a further 100 troops be sent to assist.

"Obviously, it's been a very difficult time for many months; to have this flood event off the back of others makes it more challenging," he told reporters.

The deluge is expected to ease on Tuesday, though the Bureau of Meteorology warned that rivers and water catchments may still flood as waters flow through.

It issued a severe weather warning on Monday for damaging winds and hazardous surf in NSW, as well as the risk of flash flooding and landslips.

"Some areas are expected to approach or exceed flood levels of recent events in March 2021, March 2022 and April 2022," it added.

The immediate cause of the floods has been an incessant downpour that has led to flooding in low-lying areas around major rivers.

Experts say the serial flooding in parts of Sydney has been worsened by climate change, which is affecting rainfall patterns, as well as Australia's exposure to a La Nina weather phenomenon for each of the past two years.

But the planning authorities have also been criticised for allowing Sydney's fast-growing population to spread into floodplains.

Professor Jennifer McKay, an expert on natural resources management from the University of South Australia, said state and federal governments need to develop laws to ensure that flood risk is considered when establishing new urban developments.

"Flooding occurs because people have invaded floodplains," she said in a statement to the Australian Science Media Centre.

"Climate change and the enhanced risks that come with it must be considered, and this must apply the precautionary principle to not place residents and emergency service workers at risk in the more severe events that we can expect."

Camden mayor Therese Fedeli said the scenes of residents and businesses again being forced to evacuate from submerged streets was "heartbreaking".

"I am in disbelief that we find ourselves in this situation again," she posted on Facebook.

"We are stronger together and we will get through this as a community, just like we have before."