A six-year-old boy has died after being pulled from rough seas at Ballina on the NSW north coast, bringing the total death toll from the storms that have lashed the southeast Queensland and northern NSW coast to six.

The boy was swept into the ocean at South Ballina Beach this afternoon. He was rescued by fishermen after being swept into the water while he was walking along the beach with his father and brother.

He was airlifted to Gold Coast University Hospital in a critical condition, however later died.

It is the sixth death after the wild weather claimed five lives yesterday.

Four people died after these two cars were washed from the road in Caboolture yesterday. (9NEWS)

A five-year-old boy was among the dead. (9NEWS)

A 75-year-old man died and a 68-year-old woman was pulled alive from fast moving water in Burpengary yesterday after their vehicle was washed off Morayfield Road at around 8:30pm (AEST).

It followed the deaths of four people in one incident just hours earlier.

A five-year-old boy and three adults were killed after two cars were swept away in Caboolture at around 5:30pm yesterday.

The boy was pronounced dead at the scene along with a 74-year-old man and a woman, 39.

Police said it appeared the two vehicles followed one another through the floodwaters. (9NEWS)

All were travelling in the same car.

A 49-year-old was travelling in the second car with his 21-year-old daughter and 16-year-old son and was reported missing at the time.

The younger passengers were able to flee but the body of their father was found around 100m away this morning.

Police said it appeared the cars involved had attempted to cross fast-moving water.

"Sadly, it’s unfortunate with events like this we have to highlight the dangers of driving in floodwaters," Inspector Lee Jeffries said today.

"If it’s flooded, forget it."

Insp Jeffries said the message needed to be reiterated no matter the circumstance.

"I haven’t seen anything as significant or as tragic as this," he said.

Parts of the Sunshine Coast have been hit with more than 100mm of rain in just one hour. (9NEWS)

IN PICTURES: Deadly storms smash south-east Queensland View Gallery

"I understand at the end of the day people are trying to get home to loved ones.

"I’d like to never see these types of incidents again."

A severe weather warning and flood warnings remain in place for the NSW Mid-North Coast, with the NSW State Emergency Service receiving more than 600 calls for help overnight.

Queensland Premier Annastacia Palaszczuk described the deaths as tragic and said police investigations were underway.

"On behalf of all Queenslanders we express our deepest sympathies," she told reporters last night. She described the storm cell as "off the scale".

Caboolture alone received 333mm of rain from 9am, with 277mm falling within three hours yesterday afternoon. At one point, the road to Caboolture Hospital was cut off and new patients diverted to nearby hospitals.

The Queensland State Emergency Service responded to 1800 incidents throughout the day and overnight, and there were 30 swift-water rescues. The wild weather moved south to coincide with peak-hour traffic in the capital.

Dozens of people were rescued from flood waters, train services were suspended and cars were seen floating down roads that turned to rivers.

Dams including Wivenhoe, Somerset and North Pine were forced to release water and the NRL was forced to postpone the Anzac Rugby League Test at Suncorp Stadium to tomorrow.

A 16-year-old girl was also reportedly struck by lightning last night in Burpengary, north of Brisbane. The Queensland Ambulance Service said the girl's injuries were minor and would not confirm whether it was from a lightning strike.

"This has been an extreme weather event," Ms Palaszczuk said.

Flooding in Mullumbimby. (9NEWS)

Last night she urged Queenslanders to remain vigilant, obey road closures and keep away from creeks and waterways.

"Stay out of floodwaters, make sure you are safe, and once again can I stress to families if you do not have to go out this evening, stay indoors, and just enjoy the night at home,” she said.

The wild weather will continue to lash the NSW North Coast, Mid-North Coast and Hunter Region today.

The Australia vs New Zealand rugby league Anzac Test, which was scheduled to be played last night, has been postponed until tomorrow afternoon.The NRL postponed the game after torrential rain flooded Suncorp Stadium in Brisbane.

The Sunshine Coast was also blanketed with sea foam while rivers across the region burst their banks.

Do you have a photo of the storms? Send us your photos at contact@9news.com.au - but remember to stay safe

Queensland Premier Annastacia Palasczuk urged residents in the south-east to stay home and avoid the wild weather. (9NEWS)

Ten dams in the southeastern Queensland region also began spilling as the Bureau of Meteorology warned residents to prepare for several days of bad weather.

Forecasters do not believe the east coast low will be as fierce as the storms that hammered the Hunter, Illawarra and Sydney 10 days ago.

Four people were killed, including three washed away inside their Hunter region homes, as the wild weather brought flash flooding and fierce winds. A follow up freak storm also dropped hail stones across Sydney’s inner west.

The State Emergency Service is still working through the 21,000 calls received during the horror storm and about 350 rural homes remain isolated by flood waters.

Two-three metre surf will continue to lash the coast. (9NEWS)

Beaches have been closed along the south-east coast as the BoM warns of "deceptively" powerful surf. (9NEWS)

Heavy rainfall could produce flash flooding. (9NEWS)

"We have a significant amount of resources deployed across NSW, including multiple helicopters and inter-state assistance," Deputy Commissioner Steven Pearce said.

Queensland Fire and Emergency Services advises that people should: