A woman and three children were lucky to escape serious injury when their car crashed off an Auckland motorway and rolled 20m down a bank yesterday.

The accident happened on the south-bound lane of the Northern Motorway, just before the Greville Rd offramp.

Sergeant Stu Kearns of the serious crash unit said it appeared the car lost control and "launched off the side of the motorway".

The crash brought two lanes of traffic to a standstill.

Firefighters had to cut at least one person from the wreckage and St John staff took an adult - believed to be the children's mother - to Auckland City Hospital with minor injuries.

The youngsters - one of whom was initially described as being in a serious condition - were taken to the Starship hospital.

Mr Kearns said the serious crash unit was called to the accident, but later stood down after it became known that the occupants were not as badly injured as first thought.

He believed the woman and children were "all going to be okay".

Also yesterday, two males aged 16 and 37 remained in a stable condition after a horrendous crash between a car and a bus in Christchurch on Saturday.

The smash killed two youths aged 16 and 21 and a 16-year-old female who were travelling in the car.

It was believed the two 16-year-olds - both rear-seat passengers - were not wearing seatbelts.

The bus driver and her three passengers were shocked but not injured.

Canterbury road policing manager Inspector Al Stewart said officers hoped to name the victims today, once the formal identification process had been completed.

Mr Stewart said there did not appear to be any external factors involved in the crash, and the road had been clear and dry.

"We are pretty confident there were no external factors that contributed to the crash and as such the investigation will probably focus on what was happening inside the vehicle to give us some idea of what's caused the crash."

Mr Stewart said alcohol was found at the scene but it was too early to say where it had come from or if it was a factor in the crash.

Earlier on Saturday, a middle-aged man died in a crash in the Bay of Plenty after apparently failing to take a bend and hitting some trees.

Police were called to the crash on McKee Rd, near Kawerau, about 8am.

Sergeant Ray Wylie said alcohol appeared to be a factor in the crash, and the man had not been wearing a seatbelt.

- additional reporting by NZPA