A quad biker is lucky to be alive after colliding with a deer and ending up underneath his vehicle in a forest north of Auckland today, the paramedic who treated him says.

The man, in his 50s, had been out with friends at the Sandpit -- an off-road motor sport area off Rimmer Rd in Woodhill -- when the deer came "out of nowhere".

Westpac Rescue intensive care paramedic Marcel Driessen said the man flipped over the handlebars and landed with the bike on top of him.

He sustained serious injuries to his chest and shoulder and was in "a lot of pain".

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"He didn't see [the deer], it just popped out onto the track. Maybe it got a fright or something and he collided with it.

"It really is one of those freak accidents. Nobody was doing anything silly at all, he had all his safety gear on which probably actually saved him. He was pretty lucky get off how he did, even though he's pretty serious and will be in hospital for a while I imagine."

The man was airlifted to Auckland City Hospital.

It was not clear what happened to the deer.