A former Dunedin teacher has admitted molesting a schoolboy several times while watching basketball in the late 1990s.

David Russell Bond, 66, of Mosgiel, pleaded guilty to a representative charge of indecent assault when he appeared in the Dunedin District Court this morning.

Bond was teaching at Otago Boys' High School from February 1997 to December 1999 and "on numerous occasions", police said, while the victim was watching basketball in the school gymnasium, the defendant moved in.

Bond would stand "really close" to the boy, who was between 13 and 15 at the time, and deliberately brush his hand against the victim's shorts.

He brushed the boy's penis on the outside of his clothing, the court heard.

Sergeant Chris George said the groping happened about four times and caused the victim significant stress and anxiety.

Bond refused to comment when initially confronted by police over the allegations.

It can now be revealed the teacher was also convicted of historic sex offences in 2014.

On that occasion he admitted indecently assaulting a boy at Wanaka in May 1975.

Bond was sentenced to four months' home detention and 100 hours' community work and was ordered to pay $4000 to the victim.

The defendant, who was in his mid 20s at the time, and a colleague took five males on a tramping trip to the Mt Aspiring area.

One night Bond slept beside the boy, who was woken by the teacher persistently pressing against his bottom and back.

The boy rolled away and went back to sleep but was again woken by Bond, who had put his hand inside the boy's sleeping bag.

In light of the previous conviction, Judge Kevin Phillips said the defendant would most likely be jailed when he was sentenced in August.

He remanded Bond in custody but allowed a report to be compiled to consider home detention.

"For the victim it isn't historical, it's a day-to-day fact of life," the judge said.