Australia's two most populous states are preparing for heavy rains over the weekend as authorities on Friday urged residents in flood-prone regions to avoid unnecessary travel.

Starting late Friday, an intense weather system will bring widespread rain over eastern New South Wales (NSW) and northern Victoria likely resulting in riverine flooding, the latest warnings from Australia's Bureau of Meteorology showed. Some areas could receive up to 100 mm (4 inches), about a quarter of Australia's annual average rainfall.

"Definitely a concerning couple of days ahead," Dean Storey, emergency services assistant commissioner for NSW, told ABC television.

Many rural NSW towns are already inundated with television footage showing residents moving livestock on boats. Sandbags are being offered to help protect property, emergency crews said.

The wild weather system is also likely to affect parts of Queensland and Tasmania before clearing out on Monday.

Australia's east is in the grip of a rare third straight year of the La Nina weather event, associated with increased rainfall. Sydney on Thursday recorded its wettest year since records began in 1858, with almost three months left in 2022.

Dams and rivers are almost full and more rain can trigger flash flooding as officials warned motorists to avoid waterlogged roads during the last weekend of school spring break.

"Please don't risk it. We know that floodwaters can rise very quickly. The ground is saturated, our rivers will come up very quickly ... if it's flooded, then stay away from those waters," NSW deputy premier Paul Toole told reporters.

Gunnedah, a rural town about 400 kilometres (250 miles) northwest of Sydney, could be hit with its third major flood in a month.

Devastating floods have repeatedly struck Australia's east coast this year. In March, rising waters forced tens of thousands from their homes and at least 13 were killed.