Warning Sydney Island plans to travel, wet and wild weather is expected to last through the week and flash flood warnings around New South Wales after autumn begins relatively warm.

Between Tuesday and Friday, the Sydney Meteorological Agency expects more than 100 mm of rainfall in Sydney. Predict every day at least anywhere between 5mm and 25mm.

NSW SES issued a flash flood warning to the NSW North Coast on Monday afternoon, suggesting that Rapid River Rises could pose a lot of risks.

NSW Prime Minister Chris Minns, who acknowledged the coming rains, said he “can’t guarantee” that road and public transport networks would not be interrupted on Monday morning.

"It's a lot of rain," Minnes said.

“This means we need to be able to react faster and faster when it comes.”

Sydney residents are urged to plan their routes in the week to find alternatives if necessary and allow additional travel time.

The city was wet on Sunday night, measuring 28 mm in the Obsvatory Hill during the day, 28 mm in the west of the city in Bankstown during the day, and recorded nearly 60 mm of rainfall in Terrey Hills to the north of the city.

During Sunday's 24 hours to 1pm, the NSW SES responded to 137 aid calls, most of which were between its northern region, the Central Coast and South Bukaka.

The area is still in standby mode, with constant wet weather and strong winds.

Angus Hines, a senior meteorologist at the Meteorological Bureau, said Sydney will have a “explosion” of sunlight this week, but “it will never be too far from the next rain”.

The temperature in Sydney on Monday was "on the mild side" and up to 19c. The rest of the week is forecasted, although Hines warns of consistent showers and the southeast winds will make these forecasts cooler, the highest point at 14c is predicted.

Other areas of the other east coast of New South Wales are expected to continue wet in the coming days, with the largest waterfall expected to attack outside the metro area between Newcastle and Coffs Harbour.

“For that area, we have severe weather warnings for heavy rain and damaged winds, and it’s increasing today and is expected to last until at least Thursday,” Hines said on Monday.

Slight to moderate flooding is expected along the central and northern coasts, and heavy rain is expected to fill coastal catchments.

Flooding was 100mm in gusts Sunday night and Monday morning, and flooding watches were also built for several rivers in the Hunter area, which is expected to rise as rainfall continues throughout the Central Coast on Monday.

Hines said the “extended” nature of the weather event could see rainfall for several consecutive days in northeastern New South Wales. "We certainly won't rule out the opportunity for several rivers to see major floods this weekend," he said.

The agency also issued several dangerous surfing warnings on the NSW coast between Sydney and Macquarie Port, with gusts of up to 80 km/h expected to cause massive expansion.

Hines said the "big powerful wave" recorded on Sunday suggests that disrupting surfing conditions could erode the shoreline, especially between Sydney and Seal Rock, north of Newcastle.

Hines warned that dangerous coastal conditions could increase Monday afternoon, with “dangerous” conditions precluded activities such as fishing, swimming, boating and surfing.

The temperature dropped to icy 2C on Monday, and the Melbourne also had a cold start, recording 0C at the Melbourne Airport, and the temperature in the outer suburbs dropped to the negative range. Temperatures are not expected to rise above 20 degrees Celsius, at a weekly low between 2C and 11C.

Hines said subzero temperatures will repeat Tuesday and Wednesday mornings in southern Victoria, South Australia and Tasmania - although the second half of the week will slowly climb to the "more seasonal" range.

Although NSW is ready for flooding, South Australia is still very dry, while another major dry week, “only a few millimeters of rainfall” is expected.

Hines said that 5 to 25mm of rain over the weekend in certain parts of the state since the second half of last year “is not enough to make up for the lack of rainfall.”

The weekend may be a chance to reach “slightly important things” – Hines said it would be a “step in the right direction” but pointed out that the area needs more rainfall and more often meaningfully remedy dry conditions.

Adelaide predicts a weekly low of 6C and a top of between 17c and 21c.

In the north, Queenslanders will expect clear skies and sunshine after the season starts wet. Brisbane's lowest temperature this week is expected at 15°C, with a top of 23C to 26C.