Constable Hugh Tait, based at Ranfurly, said police viewed the allegations with concern and were seeking information about the riders' identities.
''The victim felt vulnerable and intimidated, and this kind of incident leaves a sour taste for everyone,'' Const Tait said.
The man, who declined to be named, was driving a four-wheel-drive vehicle and was showing a companion some historic features in the park on December 30.
When approached by the Otago Daily Times yesterday, the man said he felt extremely vulnerable during the incident and feared for his safety and that of his companion.
Const Tait said the motorcyclists, on unregistered machines, were riding in the Mt Buster diggings area, which was fenced and off limits to motorcycling.
The park is in the Maniototo-Manuherikia area and straddles the mountain ranges dividing Otago and Canterbury.
The closest townships are Naseby and St Bathans.
The man and his companion took several photographs of the riders, and told them they should not be in the area.
When he went to leave, the motorcyclists blocked the only exit - a gateway on to a public road, the man said.
''I nudged one of the bike tyres to encourage them to move.''
After he backed away, the riders left the motorcycles lying across the gateway and surrounded the back and two sides of his vehicle, allegedly hitting it with their fists and a rock, damaging a door panel and breaking a headlight.
''I thought the windscreen was going to be smashed and I was concerned about my safety and that of my passenger so I got out of the vehicle.
''Then one of them hit me on the head with a heavy rock, bringing it down on my head, which I managed to deflect a bit by putting my arm up to save myself.''
He got back into the vehicle and eventually made his escape.
The man then sought treatment from a doctor for bruising to his arm and face.
The man said the riders were wearing full-face helmets and off-road riding gear and he believed they were in their late teens or early 20s.
The man believed two of the bikes were Honda CR250s.
Const Tait said one or more of the riders, once identified, might face charges.
