A man in his 50s has been airlifted to hospital after coming off his mountain bike in a Christchurch park.

The man is believed to have hit his head on a rock after coming off the bike in Victoria Park at around 7.30pm, Any Gray of St John Ambulance southern communications centre said.

The park, around 9km from central Christchurch, contains a series of downhill mountain biking tracks.

The area where the man was injured was inaccessible by ambulance, Mr Gray said, and a helicopter was sent to retrieve the man.

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He was airlifted to Christchurch Hospital in a moderate condition.