A tight Hamilton community is "in shock" after learning the much-loved couple from across the road has lost their two little children and are themselves in hospital.

On Good Friday, 4-year-old Arteen Mosaferi, and Radeen Mosaferi, who was 2 months old, died after a crash between two cars and two truck-and-trailer units on Desert Rd near Waiouru.

Arteen died at the scene, and his little brother died at Starship Hospital on Sunday from his injuries.

Their mother, Dr Mohadeseh Sharifi, was critically injured and their father, Siamak Mosaferi, received moderate injuries.

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Both were taken to Waikato Hospital but have since recovered and were both stable in a ward this morning.

A neighbour in their Hamilton street said together with the couple, and another neighbour, they were the first to live in the quiet cul-de-sac nearly two-and-a-half years ago.

They were all "very close", she said.

She knew the couple were going away for the weekend and believed they were heading to Wellington.

She said they were loving, "amazing" parents who often watched over as their oldest child played in the driveway.

"They're wonderful neighbours. They're amazing, it's just too cruel. It's affected us all around here. We're all very close."

Dr Sharifi and her husband migrated to New Zealand in 2009.

She graduated with a Master of Science in Chemical Engineering from Sharif University of Technology in Iran in 2007, and continued her studies at the University of Auckland, gaining a Doctorate of Philosophy studies in Chemical Engineering in 2012.

In 2016 she began working as a scientist with the dairy foods team at AgResearch in Hamilton.

Science group leader Jolon Dyer says she is an "enthusiastic and much-loved member of the team".

"We were shocked to learn what has happened to Mohy, and the terrible loss she has suffered.

"Mohy's colleagues at AgResearch will be doing all they can to support her and her family through this very difficult time."

Mosaferi completed his Master of Engineering Degree at Auckland University in 2013, before starting as a graduate structural engineer at Stiles and Hooker Architects + Engineers in Hamilton in 2014.

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Stiles and Hooker director engineer Tony Wade said Friday's tragedy was a "huge shock" to the directors and fellow staff.

"Our hearts go out to Sia, Mohy and the wider family," Wade said.

"Sia is a hardworking and valuable member of our close-knit family orientated business.

"[He] is an active participant in office social events and a proud family man."

The Serious Crash Unit and the Commercial Vehicle Safety Team are investigating the crash.

Six people died on New Zealand roads this Easter weekend, three times as many as last year.

National manager for road policing Superintendent Steve Greally is labelling the number of deaths that occurred on our roads over the holiday period "particularly tragic".

It is the highest number it has been in eight years, since 2010, when 12 people died.