Thousands of people were ordered to evacuate and more than 200 schools were closed as another round of heavy rain hit Australia's southeast on Monday, worsening "one in a hundred year" floods north of Sydney.

A days-long downpour has caused widespread flooding in the country's most populous state, New South Wales, with portions of Sydney's north-west fringes now experiencing the worst floods in decades, according to officials.

It comes less than a year after unprecedented climate-change bushfires ravaged the same area, following a prolonged drought that forced Sydney to impose water restrictions.

The state's Mid North Coast has been especially hard hit, with Premier Gladys Berejiklian calling the area a "one in a hundred-year" tragedy.

After the Warragamba Dam, which supplies the majority of Sydney's drinking water, spilled over Saturday afternoon, swollen rivers in Sydney's vast Hawkesbury-Nepean Valley were predicted to reach levels not seen since 1961.

Officials with the state's emergency services told local media that 18,000 people had been ordered to evacuate so far, with 2,000 still on the list as of Monday morning.

More than 200 schools have been closed, including some that were damaged in the flooding, according to education officials.

The Bureau of Meteorology has issued “dangerous” weather warning for Monday before the storms subside later in the week.

As the deluge spreads into the state's northwest and further north into Queensland state, rainfall records are expected to continue to fall in the coming days.

The rain and flooding, according to health officials, will delay the already halted roll-out of coronavirus vaccines in Sydney and the surrounding areas.

Australia is set to start the first major public phase of vaccine distribution on Monday, despite the fact that the program has fallen behind schedule due to supply and delivery problems.