One person is in critical condition and another is seriously injured after an assault in Christchurch overnight.

Police were called to the incident at a property on Olivine St, Shirley at about 12.15am on Thursday.

A spokesperson said there were reports of an altercation between people who were known to each other.

A St John spokesperson said they sent three ambulances, one rapid response vehicle and one manager to the scene.

Two patients, one in a critical condition and the other serious, were transported to Christchurch Hospital.

"Inquiries are under way to establish exactly what took place," police said.