Lammas was sentenced to four years and five months on 31 May after pleading guilty to a raft of offences.
They included six of indecent assault with a child under 12, five of sexual connection with a young person, and one of sexual grooming.
The offending was against three victims between 2018 and 2020.
Detective Matthew Lee said police believed there may be more victims from across Hamilton, Whakatāne and Tauranga.
“The offending may be historic with the victims likely to have been boys aged 4 -15 years old at the time of the offending,” Lee said.
“We would like to reassure those who come forward that the information they provide us will be treated with sensitivity.”
Anyone with information can contact the investigation team by calling police’s 105 number and quoting file number 201130/9428.