"Police are canvassing the area for CCTV footage and are also asking any members of the public who has information which may assist police to contact the Kumeu Police Station."
The Huapai Domain is home to several sports codes including cricket, netball and football as well as a host of social fitness clubs.
A football training session was underway nearby at the time of the alleged incident.
Norwest United Football Club chairman Thomas Epstein said the club and community were "devastated" that this had occurred in the area.
Epstein said the matter was reported to the police that evening and the club was also notified.
The matter had been taken seriously and the club had mailed its player database, coaches and managers as well as posting on both the club's Facebook and website about the incident, he said.
Epstein said he was also speaking to representatives of other sports codes as well, he said.
"We have also instigated a club buddy system for players going forward, with an adult accompanying any player under 14 should they require the restrooms.
"Even though the season has ended, we still have some teams training for tournaments."