Hundreds of residents in flood-ravaged Australia's southeast were rescued by helicopter and boat after rivers rose rapidly on Tuesday, cutting off entire towns and inundating houses, highways, bridges and farms.

Emergency crews deployed more than a dozen helicopters to rescue trapped people from rooftops in the worst-hit towns in New South Wales (NSW) state, officials said.

More than 200 flood rescue operations were conducted in the state over the past 24 hours to Tuesday morning, while 24 emergency warnings remained.

NSW Rural Fire Service said their helicopters saved 67 people, with some clinging to trees and on rooftops.

A faster-than-expected rise in floodwaters forced officials to bring forward by two hours evacuation orders for Forbes, where the rural wheatbelt town's 8,000 residents are battling their second major flooding in as many weeks.

"They can't believe this could happen twice in such a short period of time," Mayor Phyllis Miller told ABC television. "I'm an optimist in my life but this is really heartbreaking."

About 600 properties in Forbes are likely to get inundated with the Lachlan River set to match levels hit during the floods in 1952, officials said.

Australia's east is in the grip of its fourth major flood crisis this year - the latest one into its third month - because of a rare multi-year La Nina weather phenomenon, which brings more rain.

Heavy rain smashed parts of southwest New South Wales and northeast Victoria over the weekend. Cowra, which lies about 300 km (186 miles) west of Sydney, received 121 mm (4.8 inches) over 24 hours to Monday morning, the highest daily rainfall in 118 years, data showed.

Although rains have eased and blue skies returned to many inundated areas, emergency crews warned the danger had not yet passed.

"Although it is sunny out there at the moment, the waters are still flowing into catchment areas and creating severe risk to communities," NSW emergency services commissioner Carlene York said during a media briefing.

Flooded highways near several inland towns are hampering rescue operations as emergency services said they were undertaking what would be the biggest flood rescue operations in New South Wales' history.

Premier Dominic Perrottet said the federal government will deploy an additional 100 defence personnel for relief efforts. Volunteers from New Zealand have arrived while the state has also sought help from the United States and Singapore, he said.