Australia's east coast was smashed by heavy rains on Saturday, forcing multiple evacuation orders and pleas from authorities for people in Sydney to stay at home amid the potentially life-threatening flash flooding.

Most of the coast of the New South Wales state, which is home to about a third of Australia's 25 million people, has already seen March rainfall records broken with heavy rain forecast to continue for the rest of the weekend.

"It's not just the total rainfall amount, it's also how quickly and intensely that rainfall has actually fallen," Agata Imielska, a senior climatologist with the Bureau of Meteorology, said at a televised briefing.