A man is in Christchurch hospital with serious head injuries after being assaulted, police say.

The alleged attack occurred about 11pm on Saturday in Sewell Street in the suburb of Linwood.

Police are yet to release further details, saying they were speaking to witnesses as well as family of the man, aged in his early 20s.

Newstalk ZB journalist Samantha Olley said police are asking anyone with information to contact them. 

"This morning police are examining the scene and gathering evidence. Police want to hear from people ion the area who noticed anything out of the ordinary last night."