The Department of Conservation is considering prosecuting the owners of five ferrets found on environmentally sensitive Great Barrier Island.

DoC's Great Barrier area manager, Dale Tawa, said he was flabbergasted to get a phone call last Sunday from an island resident to say ferrets on leashes were being exercised on a local beach.

Mr Tawa said he rushed to Smokehouse Bay to see the sight for himself. "My heart was in my mouth."

He and another staff member boarded a boat moored in the bay to find the ferrets caged there. Those on board said they did not know they were contravening the Biosecurity Act and Wildlife Regulations.

Mr Tawa said he was told the ferrets were desexed, but he was horrified to think of the damage one ferret could do to wildlife if it escaped.

"They eat anything and everything," he said.

The boat owners received a formal warning last Monday. They left the island that day, although they had apparently planned to stay for a week.

The Auckland Regional Council issued a notice of direction to the boat owners, and is now investigating options for legal action.

ARC parks and heritage chairman Bill Burrill said the incident was extremely alarming.

"Ferrets are the ultimate scavengers and the ultimate survivors. Nothing would be safe."

Mr Burrill said the protected status of the Hauraki Gulf had been well publicised and "ignorance was no excuse".

Because of its isolation, Great Barrier Island is free of major introduced predators such as possums, ferrets, hedgehogs and Norway rats.

As a result, native species such as the brown teal, black petrels and the extremely rare chevron skink have thrived.

DoC public relations manager Ian Bradley said that under the Biosecurity Act, those prosecuted could face fines of up to $100,000 and five years in prison.

"That is how seriously we take this," Mr Bradley said.

Acting Auckland conservator Warwick Murray said the incident almost defied belief.

A discussion document on the future of ferrets in New Zealand had been under discussion for two years, he said.

"I find it distressing that pet owners, members of a recognised club, apparently claim not to have been aware of regulations governing the keeping of these potentially dangerous animals."

He said DoC strongly advised people not to take pets to sea with them.

"Experience has shown time and time again pets do escape."

It is legal to keep ferrets as domestic pets in New Zealand, but there are strict regulations governing their ownership.