Two teenage boys have saved a woman's life after her car took an icy plunge into a Christchurch Lake this morning.

The car entered a lake in the Westlake Reserve about 8.15am.

Fire and Emergency New Zealand shift manager Ian Littlejohn said two teenage boys swam into the water to help the woman involved.

When FENZ arrived the car was already fully submerged, he said.

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

Could they please contact Hillmorton High School and St Thomas of Canterbury College, and let them both know the boys would be late, 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."

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.

He was only 15 and had stepped up to help, she said.

"We are really big on citizenship and social action," she said.

"I don't think it comes any more clear cut than that."

It is the same lake wheretwo people tragically died after a car accident in September 2017.

Avneel Ram had been teaching wife Sheetal to drive when she accelerated into the lake.