Christchurch police are calling for witnesses to a nightclub assault in which a young man was seriously injured late last month.

The "physical altercation" happened inside Baretta Bar & Nightclub on St Asaph St between 11pm and 1am on the night of Saturday, April 28.

Police are urging "anyone involved or anyone who may have information about what occurred" to call Christchurch Police on 03 363 700, quoting file number 180430/2057.

Information can also be provided anonymously via Crimestoppers on 0800 555 111.