Salwa Mustafa was told New Zealand was the "safest country in world" before her husband and 16-year-old son were killed in the Christchurch terrorist attack.

Khaled Mustafa and his son Hamza were praying at the Deans Ave mosque on Friday when a lone gunman opened fire on worshippers killing 50 people.

A second son Zaid, 13, is recovering after a six-hour operation at Christchurch Hospital.

Hamza had celebrated his birthday two days before the attack and a piece of his cake was still in the family fridge, RNZ reported.

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The 16-year-old called his mother shortly after the shooting started.

"He said 'mum there is someone in the mosque shooting us," she told RNZ.

She heard the gunfire and called out to Hamza over the phone.

"He couldn't answer me. But I could hear he was trying to say something in a very low voice.

"I held the phone for 22 minutes trying to connect with him. His phone was on. He didn't close his phone.

"'Hamza, Hamza, tell me what's happening, Hamza.'

"There was complete quiet. I couldn't hear anything."

Salwa Mustafa rushed to the mosque with friends she was with, RNZ said.

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"Somebody picked up the phone and said 'sorry he can't breathe, I think he's dead'."

Hamza Mustafa celebrated his 16th birthday two days before his death. Photo / Supplied
Hamza Mustafa celebrated his 16th birthday two days before his death. Photo / Supplied

Hamza was a Year 12 student at Cashmere High School. He was one of two current students, and seven people altogether, associated with the school who were killed or injured in Friday's terrorist attacks.

Principal Mark Wilson said the school began Monday with three separate assemblies for its 2000-plus students, to acknowledge the tragedies and to give students and staff a moment to come together.

Hamza joined the school last year after his family fled the civil war in their home country of Syria.

They had lived in a Jordan refugee camp for six years before being accepted.

He was an excellent horse rider and aspired to be a veterinarian, Wilson said.

"He was a compassionate young man. Despite not being here a long time he had developed many friends."

Cashmere High School head boy ‎Okirano Tilaia‎ said his fellow students were in a "state of shock" since the attacks.

"I didn't know any of [the victims] personally, but we are all grieving. We didn't think something like this could happen so close to home."

Wilson said he was proud of how his students were responding to the tragedy.

"It is easy to be overwhelmed by the hatred, madness, darkness, in these situations.

"The message we are emphasising is the best thing you can do is turn around and show light, show love, and that can be done in the most simple thing of showing a smile to somebody.

"Hatred won't win. It is not who we are as a country or a city, our real side is being revealed in our response."