The 19-year-old was caught by a police dog patrol in the Styx Mill Reserve on Sunday afternoon.
It followed more than 10 reported sightings of the man, often doing an indecent act, over three weeks, Christchurch North Community Patrol chairman Sam Hall said.
A police spokeswoman said the man was arrested and given a pre-charge warning.
The warning is an alternative to prosecution for offenders who have committed offences with a maximum penalty of no more than six months imprisonment.
Police use discretion to not prosecute, even though there is sufficient evidence to do so.
“It’s quite ironic that the dogs found him in the dog park,” Mr Hall said.
A woman, who didn’t wish to be named, said her teenage daughter and another teenage family member were out walking on a track off Hussey Rd, when the man, with his pants down, began following them.
“When they looked around he kept hiding behind bushes, they started to jog as they felt really uncomfortable and felt something wasn’t right. It was a hot day and he had a thick hoodie,” she said.
Mr Hall said his patrol group, which has about 30 volunteers, had been monitoring the area during its daytime and evening patrols.
