A teenager was sexually assaulted by a middle-aged man in a daylight attack outside a Christchurch surf club.

Police said the 17-year-old was assaulted by an unknown man about 4pm on Sunday, May 1.

The assault happened after the teen was approached by the toilets near the South Brighton Surf Life Saving Club on Marine Parade.

Police said the man was described as tall, fat Caucasian aged between 30 and 50.

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He was wearing tan-coloured chino pants, a green shirt and a brown business jacket.

Detective Stephen Rochford said the incident highlighted the need for members of the public to be mindful of their surroundings, and to contact police if they observed people behaving in a suspicious or concerning manner.

Anyone who could help identify the man should contact Christchurch police on 03 363 7400 or Crimestoppers, anonymously, on 0800 555 111.