Two women from the same family died in a head-on crash after leaving a whanau get-together - and police are investigating whether the driver of the car that hit them crossed the centre line while using his cellphone.

The crash, one of more than 370 to have happened on New Zealand roads during the holiday period so far, happened just south of Tokoroa on State Highway 1.

The women were travelling south when their SUV and a Nissan heading north collided.

A relative took to Facebook to mourn the loss of his aunt and cousin on Boxing Day. The man said he saw the crash on the news, but didn't realise it was his family.

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"I just saw those two on Christmas Eve but they were walking the other way so I didn't get to talk to them," he said. "That was the last time I saw them."

The Nissan driver was taken to Waikato Hospital with serious injuries.

Police say part of their investigation was ascertaining whether he was using a mobile at the time of impact. A cellphone was found in the car.

Last night, the holiday road toll stood at four, including a cyclist.

Police said they were "cautiously optimistic" there would be fewer fatal crashes this year compared to 2014.

The holiday road toll period officially started at 4pm on Christmas Eve. Almost 27 hours passed before the first fatality - in Northland on Christmas Day, when a French tourist died in a three-car crash.

His female passenger and one of the other drivers suffered serious injuries, and two children were treated for minor injuries.

Initial police investigations showed the French man's rental car crossed the centre line and collided with the other vehicles.

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The toll rose to three on Boxing Day after the crash near Tokoroa, and then to four when a 57-year-old who had come off his bicycle a day earlier died as a result of his injuries.

The cyclist was not wearing a helmet when he fell.

Yesterday, emergency services were kept busy on the roads as holidaymakers left the main centres.

In the afternoon, 10 people were taken to hospital, one with serious injuries, after a three-car crash in South Waikato.

A scooter rider was in a critical condition after a collision with a car in Papatoetoe, and one person was taken to Whangarei Base Hospital by helicopter after going off the road and into a ditch in Towai, Northland.

National road policing manager Superintendent Steve Greally said he was pleased the toll was still low but even one life was too many, especially at this time of year.

"Irrespective of the numbers, the important thing is that there are families who have lost someone at Christmas," he said.

Mr Greally warned of delays on the roads as people left the main centres for their summer holidays and urged drivers to take extra caution.

"People make such bad decisions when they're frustrated and crash. They can't keep a lid on their anger," he said.

The warning comes as holiday makers hit the roads and traffic hit peak levels on routes out of Auckland yesterday.

The Transport Agency warned drivers of significant delays at the Thames turnoff. It also predicted heavy flows from Auckland to Northland, the Waikato and Bay of Plenty again today.

It advised holidaymakers to avoid the heaviest periods, such as from 10am to 1pm on both the Puhoi-Wellsford section of State Highway 1 and eastbound along State Highway 2 through Maramarua to the Coromandel Peninsula and Tauranga.

Those heading south to Hamilton and beyond should avoid peak travel from 10.30am to 12.30pm if possible.