West Coast police boarded a moving 10-tonne bus and threatened the alleged drunk driver with a Taser after a 15-minute low-speed chase.

Constable Rhys Martin said the bus first came to the attention of police about 9pm on Friday, near Hokitika.

It was travelling north slowly, with no front lights. Martin said when a police car pulled in front of the bus and turned on the flashing lights, the driver allegedly rammed the officer's car.

From there a slow pursuit began. Martin claimed the bus continued slowly toward the Hokitika Bridge and the efforts of a second police car failed to bring the pursuit to an end.

Police allege the bus driver swerved into a third police car, which was driving alongside the bus, before going over the bridge.

Martin said the man in the bus then left the driver's seat and moved down the aisle to the rear window and began gesturing at police while the bus meandered down the left lane at a speed of about 10kmh.

Officers managed to board the bus as it was moving and stop it, 1.8km after the first failed attempt to pull the vehicle over.

Martin said police aimed a Taser at the man, who gave himself up without further incident.

A 36-year-old Hokitika man appeared in the Greymouth District Court yesterday charged with assault with a weapon and being an unlicensed driver.

Further serious driving charges were likely, and alcohol was believed to be a factor, Martin said.