This morning Jack Hodgson was just waiting for his bus to school, this afternoon he's one of the teenagers who rescued a woman from a freezing Christchurch lake.

The woman came into strife after her car, with two dogs also inside, crashed into the water after one dog stepped on the accelerator.

Hodgson, 15, said the bus was nearly at his stop when he heard a woman screaming for help at the top of her voice.

"I could see the car in the water and I didn't realise at first, but two dogs were in front of the car. The woman was behind the car trying to hold it up," he said.

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"When I saw the car flip over her I got some adrenaline and jumped in after her.

"I was just like 'oh crap', no one else wanted to jump in and I thought if no one else is going to do it then I should."

Swimming in the water became incredibly difficult due to his clothing and the temperature of the water, Hodgson said, and another teen came to his aid.

Joseph, from St Thomas of Canterbury College, ventured into the water after him and brought him back to shore before rescuing the woman.

Her two dogs did not survive.

"I tried to go for the dog and the lady but I knew as soon as I got in the water I was only going to get the lady," Hodgson said.

"The dogs, I feel bad for them, just how it happened. They should just put a fence there. That wouldn't have happened if a fence was there."

Hillmorton High School acting principal Jo Hunnikin said they were incredibly proud of student Jack Hodgson, who was one of the two boys involved, and his actions.

"We are really big on citizenship and social action," she said, "I don't think it comes any more clear cut than that."

Fire and Emergency New Zealand shift manager Ian Littlejohn confirmed the two teenagers jumped in to rescue the woman.

A firefighter on the scene had described the boys' efforts as "sterling" and had a simple request for the Fenz team, he said.

It appears after the rescue the second boy to arrive at the scene simply picked up his bag and headed to school, as if his heroics were an everyday occurrence, he said.

"The ambulance checked everyone over as a precaution, as you can understand at that time of the morning it is very cold and frosty in Christchurch.

"People are very quick to criticise the youth of today, but these two young adults certainly are a credit to both their families and their school."