A Victoria Police sergeant sacked for issuing speeding drivers with smaller fines than the law required had been maintaining the practice for the last 10 years, Deputy Commissioner Ken Lay said.

The northern suburbs sergeant, who was dismissed this week, is accused of reducing the penalty for drivers by increasing a zone's speed limit on a ticket.

If a motorist was caught driving 80km/h in a 60km/h speed zone, the sergeant allegedly falsely increased the speed limit to 70km/h so the driver would be fined less and receive one instead of three demerit points.

Deputy Commissioner Lay said in a statement this afternoon the officer had recorded incorrect details on at least 250 tickets in the past year and had admitted continuing the practice for the last 10 years.

He said the sacking was the result of a "long and thorough" investigation.