Police have closed part of a motorway near Christchurch tonight.

In a Facebook post, Transport for Christchurch said the northbound Ohoka Rd off-ramp was closed on the city's Northern Motorway.

The State Highway 1 off-ramp was closed about 5.30pm because of a police operation.

The closure was causing some delays in the area, they said in the post.

"Motorists are recommended to use the Lineside Rd off-ramp instead."