A Whanganui mother was killed - along with her son - when she was momentarily distracted while driving on State Highway 3.

Jennifer Town was distracted by a passenger and didn't notice a car stopping in front of her to make a right turn.

In his report, coroner Tim Scott said Mrs Town didn't realise until the last minute that a car was preparing to make the right turn into Williamsons Line from State Highway 3 near Marton.

To avoid the turning car, she pulled to the left, lost control of her car and trailer, and travelled across the road into the eastbound lane and collided with another vehicle.

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The crash killed Mrs Town, her son Zane and a passenger in the other car, Palmerston North woman Judy Redfearn.

Mrs Town, a 41-year-old childcare worker, and 14-year-old Zane died at the scene, while Mrs Redfearn, 68, died in Palmerston North Hospital the same day.

The coroner ruled out the driver of the turning car having any responsibility.

The accident happened on May 9, 2015, when Mrs Town was driving a Holden and towing a trailer loaded with a go-kart. The car was warranted but not registered and the trailer registered but not warranted.

The other car, a Mazda driven by Mrs Redfearn's husband Fred, was warranted and registered.

After inspection of all vehicles, no faults were found to contribute to the crash.

There were three other people in the Towns' car, which was travelling toward Whanganui. They were Mrs Town's son Hayden, daughter Rachel and her son Lucas.

The driver of the turning vehicle told the coroner he was driving back from work in Palmerston North to his home in Marton and was preparing to turn into Williamsons Line.

As he approached Williamsons Line he saw the Redfearns' car approaching, which he needed to give way to.

He pulled into the centre of the road to make the turn and watched a vehicle approach from behind.

He said the approaching Holden was driven by a woman "who appeared to be looking at and talking to a passenger in the front seat".

He accelerated to get out of the way of the car as it did not seem to be slowing.

Looking back he saw the trailer the Holden was towing was in the air.

He did not see the car but the trailer appeared to be almost at right angles to the road as if it had jack-knifed. He heard a loud bang.

Mr Scott noted that a turning lane has now been created on SH3 for traffic turning into Williamsons Line. It was not there when the crash happened and he had been advised by NZ Transport Agency it was done as part of the 2015-16 safety programme.