Four people are in hospital after a string of serious crashes this weekend.

A three-year-old is in a critical condition in Starship hospital after a car rolled on Kaipara Coast Highway yesterday.

Two 15-month-old twins were also in the car with their mother and another woman.

One of the twins remains in hospital in a stable condition. The other twin and the two women were not admitted to hospital.

A witness to the accident told the Herald on Sunday he saw the car weave across the centre line before zig-zagging down the road and rolling over.

He said the crash happened on a straight stretch of road, and police said the roads were not busy at the time.

The Serious Crash Unit is investigating.

In Nelson, a motocyclist is in a stable condition in intensive care after an accident on the Whangamoa Saddle.

The 56-year-old man was airlifted to Nelson Hospital after he hit a bank near the summit of the Whangamoas around 3.30pm.

Acting senior sergeant Steffan James said speed appeared to be a factor in the crash.

"He failed to take a left-hand turn and narrowly missed a vehicle coming from the other direction," said Mr James.

Police have spoken to the driver of the car.

Mr James said there were no indications that alcohol was a factor.

In Auckland, a heavily pregnant woman was also in a stable condition in Auckland hospital after crashing into a lamppost.

Marcia Trembath was travelling on Auckland's Northwestern motorway yesterday when her car skidded into the lamppost around 10am. The lamppost then fell on the car, narrowly mising her two children, aged two and four.

Her partner Dean Delaney, a well known soccer player for Mt Albert-Ponsonby, told the Herald on Sunday she had a fractured spine but there were no known injuries to their unborn baby.

"She is 35 weeks [pregnant], and that is looking okay still," Delaney said.