As Robbie Cattle reached a terrified mum and two young kids who were treading water next to their sinking car, he told them "don't worry, I'm going to help you".

"One of the kids was screaming and she was panicking," he told the Herald.

"I said 'don't worry, I'm going to help you, I'm here now'."

Moments earlier he'd be driving down a Bucklands Beach Rd when he came across a group of people screaming on the side of the road. A car was submerged in water about 10 to 15m offshore.

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While the exact details of how it got there are unclear, neighbours believed the car was parked in a steep driveway on the other side of the road but the handbrake might have failed.

The car entered the water on Sunday night. Photo / File
The car entered the water on Sunday night. Photo / File

They say a man, believed to be the woman's husband and children's father, desperately tried to stop the car as it raced down the driveway on Sunday night. He wasn't able to stop it and it reportedly hit a parked car, flew across the road and landed in deep water.

Cattle said he just reacted to the situation, pulling his clothes off as his partner told him "go, go, go".

"She was on the phone to 111 and I was already pretty much half naked," he said.

The trained divemaster leapt into the cold water, swimming the 10m-15m distance to the screaming family who had managed to get out of the car.

Robbie Cattle cut his feet during the rescue. Photo / Supplied
Robbie Cattle cut his feet during the rescue. Photo / Supplied

The mother was cradling her children and treading water.

When he reached the struggling trio he pulled the mother higher up above the waterline.

"She was starting to go under the water," Cattle said.

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"Then I put her on my chest and she was holding them [the children]".

He swam back to shore as fast as he could with them in his arms, as the mother thanked him repeatedly.

Police later told him the water was 4 or 5m deep.

But Cattle's pretty humble about his actions.

"I didn't really do that much, I only just got them to shore," he said.

He later had to seek medical attention for his cut feet. But he says it could've been worse.

"I'm just glad that everyone got out alright and that we're not reading a different article."

The man was seriously injured and taken to hospital.