He said offending took place in Wellington and Wairarapa over at least three decades but the victims that the police knew of so far, were not from Wairarapa.
The offences occurred from 1973 to 1991 and possibly later than this, against children aged seven to their early teens, he said.
"I believe there may be other victims and I urge them to get in contact with me."
Trethewey plead guilty to 42 charges, which included attempted rape and sexual violation against children under twelve years of age.
He was sentenced to 12 month's home detention and also ordered to pay $30,000 in reparation for emotional harm to his victims.
Mr Bealing said police laid charges after receiving a complaint from a female in 2011.
"An investigation located a further nine women and one man who also made complaints."