Rapidly rising rivers swamped swathes of rain-lashed Sydney on Monday, forcing thousands to flee "dangerous" floods as the city's largest dam spilled torrents of water.

On the third day of torrential east coast rains, emergency workers said they had rescued more than 80 people since the previous evening.

Many people had been trapped in their cars trying to cross flood-swept roads or were unable to leave homes surrounded by rising waters.

Australia has been at the sharp end of climate change, with droughts, deadly bushfires, bleaching events on the Great Barrier Reef and floods becoming more common and intense as global weather patterns change.

Higher temperatures mean the atmosphere holds more moisture, unleashing more rain.

About 32,000 people were ordered to evacuate or be ready to flee across New South Wales, the emergency services department said, with the army sending 100 troops to help operations in the storm-battered state.

"The ground is saturated, the rivers are fast flowing, the dams are overflowing," said State Emergency Services commissioner Carlene York.

"It is particularly dangerous out there," she said at a news conference.

Mud-brown river waters transformed a large stretch of land into a lake in the southwestern Sydney suburb of Camden.

Roads disappeared into the waters and mobile homes stood in knee-high water, at least one toppled on its side, television images showed.

Large volumes of water gushed from the Warragamba Dam, which has been spilling excess water since Sunday.

The huge concrete dam lies on the western outskirts of Sydney and provides most of the city's drinking water.

Heavy rains in New South Wales may persist for at least another 24 hours, forecasters said.

'Becoming more common'

The wild weather whipped up drama off the Sydney coast, as rescuers battled to help the 150-metre Portland Bay cargo ship with 21 crew which lost power in heavy seas.

The Hong Kong-registered bulk carrier is double-anchored, a police spokesman said.

A plan to airlift crew members off the boat with two helicopters had been delayed because of safety concerns, he said.

Two tugboats were headed to the ship to take it further offshore.

Australia's east coast has suffered repeated flooding in the past 18 months.

More than 20 people died only in March this year as floodwaters lapped at rooftops and torrents swept cars off roads.

The current weather system over Sydney is being fed by warm, wet air from near the equator, said Kimberley Reid, an atmospheric scientist at Monash University.

Rainfall in eastern Australia is highly variable, making it hard to pin this event to climate change, she said.

"However, our research of the March 2021 Sydney floods found that similar events over Sydney were likely to occur 80 percent more often by the end of the 21st century."

Australia must prepare for more regular flooding events, New South Wales Premier Dominic Perrottet told a news conference.

"There is no doubt these events are becoming more common," he said.

"Governments need to adjust and make sure that we respond to the changing environment we find ourselves in."