MELBOURNE — People in parts of Sydney's northwest were ordered to flee their houses in the middle of the night as heavy rains continued to batter Australia's east coast Sunday with fast-moving waters causing widespread destruction throughout the region, Reuters reported.

Flooding risk and evacuation warnings were in place for about 12 areas in New South Wales (NSW), Australia's most populous state with 8 million people, with rivers swelling and rainwater accumulation posing a danger.

"It's not just the rain which is causing the devastation," Jonathan How, senior meteorologist with the Bureau of Meteorology, told the ABC News state broadcaster. "It's strong winds as well."

How added that the heavy downpour is set to continue for the rest of Sunday in Sydney and throughout the state, with some areas expected to get up to 200 millimeters (7.9 inches) of rain.

Emergency services said they had received about 600 calls overnight asking for help; more than 60 of those were pleas for rescue from floods.

Television and social media footage showed fast-moving water unmooring houses, engulfing roads, breaking trees and damaging road infrastructure.

The extreme weather was affecting Australia's COVID-19 vaccine delivery to Sydney and throughout the state and disrupting the country's plans to deliver the first vaccine doses to almost 6 million people over the next few weeks.

NSW Premier Gladys Berejiklian, who is expected to hold a briefing later on Sunday, said the weather system moving through the state could be a one-in-25-year rain event.

Berejiklian said the downpour across the state was worse than initially expected, especially for low-lying areas in Sydney's northwest. "Yesterday, we were hoping it will only be a one-in-20-year event, now it looks like a one-in-50-year event," Berejiklian said at a televised briefing. Berejiklian said another 4,000 people may still be asked to evacuate.

Several major roads were closed across the state while many schools called off classes for Monday. The flooding comes in stark contrast with the devastating bushfires that struck Australia in late 2019 and early 2020, when nearly 7 percent of NSW land was scorched.

Several dams, including Warragamba, Sydney’s main water supply, spilt over causing river levels to surge. Meteorologists said the downpour is set to continue for the rest of Sunday, with some areas expected to get up to 200ml of rain. — Agencies