Mr Brazil said the investigator — described as a "tall slim man with short white-grey hair" — was spotted by his mother and sister striding on to the property and heading towards the back of the section.
When challenged by his mother and sister, the man refused to identify himself, Mr Brazil said.
"He just kept saying 'I'm looking for Jack'.
"He was just really arrogant ... It was just quite unnerving."
Mr Brazil, who was not there at the time, believed the man might have suspected he was inside a caravan parked at the back of the property.
His mother and sister shut the door in the man's face, and he eventually left without serving a notice, but he remained concerned by the tactics, which left him feeling "quite sick".
Ms Young, contacted by the ODT yesterday, confirmed the man involved worked with her, but declined to elaborate.
"I can't help what they say. There's nothing I can say about that."
OMV New Zealand staff were asked yesterday to confirm it was using Cargill Consultants Otago, and whether it shared any of the concerns outlined by Mr Brazil.
Australasia senior vice-president Gabriel Selischi, in a statement, would only say the company "fully respects and operates within the laws of New Zealand".
"I can confirm that we only engage qualified and trained professionals on the basis they also fully respect and work within all legal requirements in any undertakings for OMV."
The company's priority remained "the safety of all our staff and contractors working on our sites and the repeat illegal actions of these individuals puts the health and safety of our people and the activists themselves at risk".
Mr Brazil said New Zealand was lucky to have the "human rights safety net" it did, to protect against the kind of abuses committed against activists overseas, but the actions of the private investigators were still a concern.
The public controversy over the activities of another firm, Thompson and Clark Investigations Ltd, in targeting activist groups, showed how far that could go here, he said.
"I think it's pretty despicable."