''The rocked up in the anchorage and they were pretty jolly. At some stage they started letting off fireworks, then the inevitable happened and the island caught on fire. It started small but grew quickly,'' the boatie, who did not want to be named, said.
The launch was close to shore and tucked into a corner of the bay, so he couldn't be sure whether the men were letting fireworks off from the boat or the shore.
''They rowed over and tried to put it out with buckets of water, but they couldn't reach the top part of the island.''
The boaties' actions were ''pretty dumb'' but he gave them credit for trying to put the fire out.
''They could've just buggered off and hoped no one had seen the boat's name.''
Fire authorities started preparing a major response which could have included evacuating the homes on Moturua and transporting firefighters and equipment to the island by boat.
Once it became clear the wind was blowing flames and embers away from the main island, they instead opted to monitor the fire overnight and put it out by helicopter at first light the next morning.
The boat owner, believed to be from the Kawakawa area, was interviewed by Department of Conservation rangers the following morning.
Northland principal rural fire officer Myles Taylor said the current heatwave and spate of major fires meant staff were too busy to follow up on the incident immediately, but they knew who was responsible and would catch up with him later.
It would be up to the top levels of Fire and Emergency NZ to decide what action to take. The degree of recklessness, and the possible consequences, meant prosecution would have to be considered.
If the blaze had spread to the main island, just 15m away, Taylor believed it would have wiped out most, or all, of its vegetation, along with many of its endangered birds.
Over the past 10 years Project Island Song has re-introduced toutouwai (North Island robins), pōpokotea (whiteheads), tīeke (saddlebacks) and kākāriki (red-crowned parakeets) to Moturua. Kiwi also live on the island.
Meanwhile, a 25-year-old Kerikeri man has been charged with arson in relation to a fire four days earlier in Rangihoua Historic Park on Purerua Peninsula, also in the Bay of Islands.
He is alleged to have set off a home-made explosive device which ignited vegetation on the flanks of Rangihoua Pa, next to Marsden Cross. He is due back in the Kaikohe District Court on February 8.