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.