He approached the trio and tried to engage them in conversation.
When Sercombe asked the girls if he could join them, they said "no" and told him to go away.
Sercombe obliged, but not for long.
He returned to the teens shortly after and made "inappropriate suggestions" to them, a police summary of facts said.
They, again, told him "no", swore at him and then collected their gear and left the area.
The girls were shaken and upset by Sercombe's actions, the summary said.
He then wrapped a towel around his waist and left the beach.
Meanwhile, one of the girls contacted her mother, who then phoned police.
Officers located Sercombe driving near the beach shortly after.
He was stopped and when arrested he still had only the towel wrapped around his waist.
In explanation, Sercombe admitted to police that he had approached the girls but denied asking if they wanted to touch his penis.
He said he believed it was not an offence to walk along the beach naked.
In court, he pleaded guilty to a charge of doing an indecent act in a public place.
Judge Lynne Harrison convicted him on the charge, referred the matter to restorative justice and ordered a pre-sentence report.
Sercombe will be sentenced on April 7. In the meantime, the judge ordered him to not go within 50m of any public beach in New Plymouth, Waitara or Bell Block.
