Police have issued a safety warning for motorists using State-Highway 65 in the upper South Island, after a sting revealed drivers are ignoring road rules.

Twenty-one people were stopped for crossing the centre line over an eight hour period on Monday.

That came just a day after 14 people were stopped for the same reason.

Acting Inspector Leairne Dow said the majority had at least one wheel completely over the centre line.

She said the road is part of the current route between Picton and Christchurch so drivers need to obey road rules.

"You just have to watch out for yourself on the road, watch out for other people on the road, and drive to the conditions yourself to make sure you're safe when travelling."

She said offenders all appeared to be in a hurry.

"It's a combination of locals, tourists wanting to get places quicker, taking the easy way out and not taking enough attention on the road I think."