A blaze started by an explosion set off on a Bay of Islands beach has swept through part of a historic reserve known for its large kiwi population.

The fire started just before 9pm on Saturday at Rangihoua Bay, on the Purerua Peninsula, and quickly spread up a steep hillside in Rangihoua Heritage Park.

Some witnesses believed the explosion was caused by a petrol bomb, others that the device was made from a large quantity of fireworks. Firefighters described it as a home-made incendiary device.

Flames race up the hillside towards Rangihoua Pā after the explosion. Photo / Vanessa Owen

Purerua residents Richard and Vanessa Owen were moored in their boat about 200m offshore when they noticed something unusual on the beach near Marsden Cross, a monument marking the site of New Zealand's first church service in 1814.

''Vanessa knew it wasn't kosher so she kept an eye on them with binoculars,'' Richard said.

At first she thought the four men were digging a hole in the sand for a beach barbecue and they were acting surreptitiously because of the fire restrictions.

Three men then retreated to the cross while the fourth walked backwards along the beach away from the hole.

She thought he was running out a long line prior to setting it but she now believes he was laying out a trail of petrol.

She was fetching a drink when she heard a huge explosion. Racing back on deck she saw flames shooting into the sky and nearby trees.

Flames shoot into the sky seconds after the explosion. Photo / Vanessa Owen

As the fire raced up the hillside Richard Owen called 111 and Vanessa Owen phoned a friend who lived nearby.

With the men needing to walk 15 minutes back up to the road she thought her friend could intercept them at the carpark before police arrived. Kerikeri police station is a 40-minute drive from Rangihoua.

When her friend arrived two of the men had already left by car but the others had just made it up the hill. One of the men suffered an apparent seizure so St John Ambulance and a doctor who lived nearby were called.

The most upsetting thing was hearing the ''sounds of kiwi screaming'' from the burning hillside, Vanessa said.

''We're familiar with the bay and the sound of kiwi calling. Those were definitely distress calls.''

It was impossible to identify the men through her binoculars because all four had baseball caps pulled down over their faces.

However, she did get a clue when one man pulled down his shorts to moon her. The whiteness of his buttocks made it ''very clear'' he was Pākehā.

The outcome could have been much worse, Vanessa Owen said.

''If it wasn't a completely windless night it would've been a disaster.''

A Salt Air helicopter from Paihia and a Rural Fire appliance from Kaikohe at Rangihoua Bay the following morning, with Marsden Cross on the right. Photo / Supplied

Deputy principal rural fire officer Michael Champtaloup said seven fire appliances and about 30 firefighters from Kerikeri, Kaeo, Kaikohe, Kawakawa and Okaihau responded.

Given the cause of the blaze firefighters waited about 500m away until they could be assured it was safe.

There was no road access but four-wheel-drive trucks from Kaikohe-based Rural Fire were able to get down to the beach and pump sea water to douse the flames.

Champtaloup said helicopters weren't used at night for firefighting but a Salt Air chopper from Paihia spent about two hours on Sunday morning using a monsoon bucket to put out hot spots on a cliffside which was too dangerous to access from the ground. Rural fire officers would keep checking for flare-ups.

He described the cause of the fire, which covered about 1ha on the side of Rangihoua pā, as ''some sort of home-made incendiary device''.

Kerikeri fire chief Les Wasson said the area covered by the fire wasn't large but the steep terrain and the area's historical significance made it concerning, as did the risk of the fire spreading over a ridge.

The fire-scarred hillside the following day. Marsden Cross can be seen at the right. Photo / Owen Smith

Police did not want to comment while their investigation was continuing. They did, however, confirm the fire was not accidental and that some sort of device was involved.

Rangihoua Heritage Park includes the site of New Zealand's first European settlement, the site of the first Christian service on New Zealand soil and the pā of Ruatara, the chief who invited Samuel Marsden and his missionaries to the Bay of Islands. The area is also known for its high density of kiwi.

The park is about 35km northeast of Kerikeri by road and combines land owned by DoC, Ngāti Torehina's Rangihoua Pā Native Reserve Board and the Marsden Cross Trust Board. It opened in 2014 on the 200th anniversary of the missionaries' arrival.

Purerua Peninsula is home to some of the most expensive real estate in New Zealand — including the lodge where former US president Barack Obama stayed last year — but no homes were threatened by Saturday's fire.