A man chased his ex-partner's car all the way to the police station - then assaulted her new boyfriend in full view of officers.

Brendan Antony Lee Turtle had been separated from his partner for a few months after a five-year relationship. They have one child together.

Turtle was driving in Whanganui East on March 30 when he spotted his ex-partner and her new boyfriend in another car, police prosecutor Sergeant Stephen Butler told Whanganui District Court on Tuesday. Turtle overtook their car and stopped in front of it, "trying to block its path", Mr Butler said.

The car continued, and Turtle gave chase.

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At one point he pulled alongside their car, trying to make it stop, then while on the Dublin St Bridge he tried to force their car into the side of the bridge without the cars touching.

Turtle then threw bottles at the other car.

The other car continued towards the police station and the vehicles arrived within a second or two of each other and came to a "heavy stop" outside the station as a patrol car was heading out.

"The defendant went straight up to the victim and punched him several times as police went to intervene," Mr Butler said.

The ex-partner tried to get between them to stop the fight, and Turtle punched her in the side of the head in the process.

Police separated the group.

Turtle pleaded guilty in court to dangerous driving, possessing cannabis oil, and two counts of common assault.

Judge David Cameron remanded him to June 15 for sentencing.