• Slideshow: Undie 500 heading our way
Police set up a drink-driving checkpoint at the northern boundary of Oamaru from about 5.30pm, stopping all vehicles entering the town.
Undie 500 vehicles who had open bottles or cartons of liquor were told to empty them out or put them in two skips put at the checkpoint for the occasion.
Students Against Drink Driving were there, handing out packs which advised drivers in the rally of the liquor bans in Oamaru, to respect the town and behave.
The first of the decorated cars started to arrive about 5.45pm.
Earlier, police were pulling over drivers leaving Ashburton, for a warrant of fitness check on the road between Christchurch and Ashburton.
Police have already warned participants they will not be able to use diversion if they break the law.
The event involves a student pub crawl between Christchurch and Dunedin in cars worth under $500 and has in the past caused headaches for Dunedin police left to control riotous behaviour at their end over the weekend.
Participants traditionally decorate their cars for the occasion.
This year one features a portrait of Dunedin mayor Peter Chin, while another has been made to look like a panda and another looks like a case of Double Brown beer.
Dunedin and Clutha area commander Dave Campbell said there would be a boosted police presence this weekend and anyone arrested would be declined diversion, leaving them with a criminal record.
"We're encouraging those involved to enjoy themselves within the bounds of the law," he told the Otago Daily Times.
Last year the official Undie 500 was cancelled, but a rogue event in its place ended with dozens of students and non-students being arrested following rioting and drunken behaviour.
