Pet owners are increasingly bringing pet cats and dogs to pest-free islands, where they can wreak destruction on fragile native animals, the Department of Conservation says.
In the Bay of Islands, ranger Helen Ough Dealy said dogs were often brought onto the protected islands, particularly during summer. But owners had also been caught walking their cats on leashes, and with cats on a paddle board.
Rangers were also finding more and more boats moored at pest free islands with cats living on board, despite the risk of them reaching the shore.
Ough Dealy said just one cat or dog could do considerable damage to precariously recovering bird or insect populations in a very short amount of time, by killing or injuring the animals or destroying their nests.
"Two or three years ago there was a report of somebody with a sulphur-crested cockatoo, which was a pet of theirs, which they had brought on their boat and then had taken it onto the island.
"The issue there isn't so much that bird killing other rare species, it's that they may be bringing disease with them."
One cat that fell overboard from a boat was not reported, and spent 4.5 months running wild on an island unchecked.
"The owners thought that it had drowned, and therefore it wasn't a problem as far as they were concerned. But it was an island which had tīeke, saddleback, which had just been reintroduced; they feed on the ground at times, they're not just up in the trees or flying, and so are very vulnerable.
"[The cat] actually came into a restaurant area because it had caught its paw in its collar, so we saw it, and were able to catch it and take it off the island. They can and do survive, they fall overboard and then swim ashore. And you have to ask, what was that cat eating during that time?"
Ough Dealy said some owners ignored the rules on the protected islands knowingly, some were ignorant about them, and many believed their pet was not the type to chase wildlife or that it would not be able to do any damage on a leash.
Owners often would not spot the wildlife before their pet got it and even the presence of predators could cause stress to delicate breeding populations and scare some birds away from their eggs.
"The New Zealand dotterel, for example, there's less than 2000 left in the world, and we've got a large part of that population up in the Bay of Islands, and for a dotterel to be scared off the nest or go away from the nest and leave the eggs vulnerable to either heat or cold, that means that that breeding season is over for that bird.
"So you've got to wait for the next year for that bird to produce more young," she said.
A survey done on Urupukapuka Island, near Russell, had shown visitors did not want dogs there because they wanted to enjoy the quietness of the islands.
Ough Dealy said dogs landed onto beaches from boats were almost always brimming with excitement, and she had seen them get loose time and again.
"One incident that sticks in my mind is watching a very large dog being brought onto the beach in a dinghy, and then a small child who must have been about five years old, being expected to keep that dog under control on the beach - and it would have been a very large dog even for an adult to handle.
"There was just no way that that was going to happen, the dog very quickly got away from the child, and dashed away up the beach."
The rising numbers of pets brought to the islands was like a kick in the guts when huge efforts were being put into recovery efforts, she said.
"It's very upsetting, because a lot of people don't realise the amount of effort to protect these rare species. Not just the hours and hours that the Department of Conservation staff do, in trying to protect these rare species - but it's the community groups who fundraise and go through all the translocation processes.
"They can impact and damage all the work that's being done to try and restore these islands, and the amazing taonga which are out there... to have their passion and their work and so on potentially ruined very quickly, it is very upsetting to see that."
Richard Robbins is general manager of Project Island Song, a partnership to restore birds, plants and native animals to an archipelago of seven protected islands in the Eastern Bay of Islands that have been pest free since 2009.
He said people were encouraged to visit the islands and enjoy them, but not to bring pets, which could undo so much of their hard-won gains.
"We have volunteers who give thousands of hours a year to help restore the islands, and there are significant partners; hapū, the project, and the Department of Conservation, it's just a huge investment.
"The islands are special, the work we're doing is to enhance and protect the islands forever. People need to appreciate that we've got nesting dotterels, kiwi, wētāpunga, and there's other species we plan to release in the future."
The group have about 16 species they plan to reintroduce to the island, and have already released the saddleback, North Island robin, brown teal, whitehead, kākāriki and Duvaucel's gecko.
On 9 December, the group also released 128 wētāpunga, the first of a programme of reintroductions of the massive insects that they hope will carry on for another three years.
"But if pets are there and we can't keep things safe, they could have an effect on whether future releases go ahead," Robbins said.
He said many conservationists had beloved pet cats or dogs and understood the bond people had with their pets, but a protected island was not the right place for them.
Ranger Ough Dealy said she believed owners taking their pets to the islands probably will not have considered that the life of their pets was also being put at risk. The Dog Control Act allows for the destruction of dogs that attack animals, as well as steep financial penalties and even jail time for owners.
"Keep your pets at home, they're hunters, that's how they are. You're risking the life of your dog, isn't it better to look after your dog."