Neighbours who live across the road from the holiday park last night told the NZ Herald that the Mongrel Mob was out in "full force" on Wednesday night.
Gang members were partying at a home on nearby Raukura St and the residents saw a blue or grey van and black hatchback vehicle parked outside the reserve next to the camping ground between 9.30pm and 10pm.
The neighbours did not recognise the occupants as members of the local Mongrel Mob chapter, but at least two wore patches and appeared to be having a meeting.
Another man joined the group after walking through the reserve.
Detective Inspector Mark Loper said the attack was one of the worst assaults on a child he had seen in his 28 years in the police.
"The attack was brutal, on a young defenceless child who should have been spending Christmas on a dream holiday with her family. Her parents are absolutely distraught."
Mr Roper refused to comment on what the neighbours had seen.
But he said police were planning to interview known offenders living in the area, and a team of 30 staff from Taupo, Rotorua and Waikato were examining the crime scene for fingerprints and DNA.
Police were visiting local businesses and taking CCTV footage from Wednesday evening.
He said there was no change in the serious condition of the 5-year-girl in hospital, but believed she would make a full physical recovery.
Mr Loper said the girl's injuries were "significant and heart-wrenching".
"The nature of her injuries leads us to believe that the attack was sexually motivated."
Club Habitat manager Sylvia George, said she felt sick the assault had happened at the camping ground, but people often walked through the property as it was a shortcut between State Highway 1 and the town.
"I've been working here for quite a while and this is the first thing we've kind of had ... someone brutally attacked."
The family had booked to stay for one night and planned to walk the popular Tongariro Crossing.
Ms George said four other campervans were parked near the caravan, but only one camper had heard the screams of the mother.
Turangi locals condemned the attack. Tongariro Chartered Club bar worker Terry Downs said it was disgusting. "It's terrible. It's just deplorable. I can't even find the words."