"We had to swim 400m to the road." All the photos from NSW's once-in-a-generation weather.

New South Wales is in the midst of a 'one-in-100-year' weather event, according to the MidCoast Council, as relentless rainfall forces residents in flood-hit areas to evacuate from their homes.  

The Bureau of Meteorology expects Monday to bring the worst flooding event to the area northwest of Sydney since November 1961, nearly 50 years ago.

Floodwaters are expected to rise to major levels on Monday morning and inundate places such as Windsor, Pitt Town, North Richmond, Freemans Reach and Colo.

The Hawkesbury river is predicted to reach peaks of up to 15 metres and the State Emergency Service says homes and properties will be flooded, some up to roof height. 

Flooding along the Hawkesbury and Nepean rivers comes after the Warragamba Dam spilled over, prompting some concern.

Parts of Penrith and other areas along the Nepean were ordered to evacuate on Sunday as NSW battles devastating floods after days of unabated rain.

BOM's Agata Imielska said the severity of rainfall in greater Sydney would ease on Monday but the mid-north coast would continue to be drenched and inland NSW would be pounded by rain.

Parts of Port Macquarie, Taree and nearby towns have also flooded, with the heavy rain expected to keep falling on NSW until Wednesday morning.

The federal government's natural disaster arrangements have been activated for 18 local government areas across NSW.

NSW's SES says it will work beyond Easter on the post-flood clean up effort.

The photos below show some of the devastating damage caused so far.

This house was spotted floating down the Manning River at West Taree on Saturday. Image: Mid North Coast NSW SES. 

NSW Surf Life Saving in the Mid-North Coast helping with rescue efforts on Saturday. Image: NSW Surf Life Saving.  

Parramatta ferry wharf overflows and floods due to continuous and heavy rain on March 20, 2021 in Sydney, Australia. Image: Getty.  

Heavy flooding is seen in the Sydney suburb of Schofields on March 20, 2021. Image: Getty.  

A road closure at Yarramundi Lane in Richmond on Saturday, March 20, 2021. Image: AAP. 

A person walks through fast moving flood waters at Cattai Creek on March 20, 2021 in Maraylya, Australia. Image: Getty.  

The overflowing Nepean River at the Penrith weir is seen on March 20, 2021. Image: Getty.  

A petrol station at Windsor. Image: Getty.

Image: Getty.

Image: Nine.

Image: Nine.

For emergency help in floods and storms, call the NSW State Emergency Service on 132 500. In life-threatening situations, call triple zero (000) immediately.

-With AAP.

Feature Image: Mid North Coast NSW SES/Twitter.