Former officer pleads guilty to having sex with a teenage girl.

A former policeman this morning admitted charges of having sex with a teenage girl.

Dean Matthew Tapper, aged in his 40s, entered last minute pleas of guilty to three counts of sexually penetrating a girl aged 13 or over and under 16.

The pleas came at the scheduled start of a District Court trial.

The offences were committed between May and December 2012 in Midland against the same girl.

Tapper, a former senior constable, resigned from WA Police in April 2013 after being stood down from operational duties.

This morning, Tapper's defence lawyer sought an adjournment to obtain a pre-sentence report.

The court was told the issue would be whether the offences warranted an immediate or suspended term of imprisonment.

But Judge Patrick O'Neal queried why a report was needed, given Tapper had no record of similar offences and youth was not a factor.

Judge O'Neal adjourned sentencing until Wednesday. Tapper was released on bail, which was not opposed by the State.