Two people have been rescued from a car swept up in floodwaters and others from a home on the NSW south coast as severe weather threatens to wreak havoc across eastern Australia.

It came amid a warning worse might be to come as the focus of the wild weather shifted to Sydney, Newcastle, Wollongong and Brisbane later on Wednesday.

Schools were shut and people urged not to drive in Swan Hill, on the Victoria-NSW border, on Wednesday after it copped more than 85 millimetres of rain in less than a day.

“Due to the heavy rain, the front and many other parts of the school grounds are currently flooded and many connecting roads into the school. There are several road closures around the school,” St Mary’s Primary School Swan Hill posted on social media on Wednesday.

“A number of our staff are also unable to travel safely to work at this stage. For safety reasons, the school will be closed this morning.”

There is also flooding at Deniliquin, in NSW’s south-west, where emergency services said more than 20 homes had been affected after more than 100 millimetres of rain in 24 hours.

Fire and Rescue NSW zone commander Stewart Alexander said crews were “working non-stop” responding to “job after job after job”.

“We haven’t got a clear number as yet, but there are many, many homes [affected] — upwards of 20, we believe at this time, that have been reported,” he told the ABC.

“We’re doing one job and as soon as we’ve completed an incident we are immediately put on to another.”

Heavy rain on Wednesday morning prompted the two flood rescue missions at South Nowra and St Georges Basin, the NSW State Emergency Service said.

Areas of the Illawarra region and south coast had copped up to 200 millimetres of rain since 9am on Tuesday, Bureau of Meteorology senior meteorologist Angus Hines said on Wednesday.

On the south coast, the NSW SES’s Ulladulla unit reported even higher falls – with 369 millimetres at Porters Creek Dam.

A watch and act warning was issued for Lake Conjola as the lake broke its banks early on Wednesday, after rising more than a metre overnight. The SES warned water levels were likely to rise further.

“We have had our daily quota already! Porters Creek Dam has overtopped and is sitting .28 metres above the dam spilling into the Clyde River. Communities downstream please be aware that the river will rise,” the unit posted on Facebook on Wednesday morning.

The rain isn’t expected to stop anytime soon. Hines forecast another 100 millimetres, meaning more areas will likely have totals topping 300 millimetres.

“This is enough rain to cause surface and flash flooding, inundation of homes and properties,” Hines said.

“This rain combined with strong to damaging wind can also bring down trees and possibly cause power outages.”

Thousands of south coast homes were also without power on Wednesday afternoon. They included about 2100 in Batemans Bay and 700 in the area from Narooma to Tilba Tilba.

More heavy rainfall, thunderstorms, large hail and damaging wind gusts are expected to escalate throughout the day and into Thursday.

“The storm focus will shift this afternoon to the eastern half of [NSW], including Sydney, Newcastle and Wollongong,” the BOM warned in a release.

“High-end supercell thunderstorms are a possibility this afternoon and evening through the east, particularly over the north-east, with a risk of localised destructive winds and giant hail.”

NSW catchments including the Peel and Namoi rivers are on flood watch while minor to isolated major flooding is forecast in Victoria for the Otways, Surf Coast and Wilsons Promontory.

“Heavy rainfall is likely through parts of the south, including Gippsland and the Otways, with the risk of both flash flooding and riverine flooding,” the BOM said.

“Thunderstorm activity will continue in the north, with possible heavy rainfall.”

Parts of southern Queensland and northern NSW are bracing for hail more than five centimetres in diameter and destructive winds of more than 125km/h. The forecast is current for areas such as Toowoomba, Dalby, Moree and Tamworth.

“Severe thunderstorms with locally heavy rainfall, damaging winds and large hail are likely in southern and south-east Queensland today, and possible around Brisbane and the Gold and Sunshine Coasts,” the weather bureau said.

“There is also potential for isolated very dangerous isolated thunderstorms with locally giant hail and destructive winds during this afternoon and early evening.”

There are also flood warnings in Queensland for the Moonie River and Wallam Creek, Mungallala Creek and the Paroo and Barcoo rivers.

At the other extreme, severe heatwave conditions in north Queensland are expected to build along the east coast and extend south before easing on Friday.

Some far north Queensland centres have had a string of days in the mid-to-high 30s. They are forecast to continue until at least the end of the week.

– with AAP