A new book has revealed rare historical gems buried in New Zealand's national archives, including a bizarre WWII plan to create a "tsunami bomb" and military files detailing supposed UFO sightings.

Author Ray Waru said he wrote "Secrets and Treasures" to highlight the material publicly available at Archives New Zealand in Wellington -- where almost 100 kilometres (62 miles) of shelf space is crammed with historical artefacts.

"It was totally overwhelming at the beginning," he told AFP.

"I knew I wanted to get in the important things, the Treaty of Waitangi (New Zealand's founding document), the Declaration of Independence, the women's suffrage petition, and a few other things.

"But once you start digging, one story leads onto another and I'd just follow my nose."

The suffrage petition Waru refers to contains 36,000 signatures and was dramatically unfurled on the floor of the New Zealand parliament in 1893 by supporters of women's right to vote.

Stretching for almost 300 metres (980 feet), the petition, currently undergoing restoration, proved successful and led New Zealand, then a British colony, to become the first country in the world to grant women the vote later that year.

Alongside notable historical documents, such as a letter written by explorer captain James Cook before his final voyage, are curiosities like "Project Seal", a top-secret US-New Zealand attempt to create a doomsday device to rival the nuclear bomb.

The project was launched in June 1944 after a US naval officer noticed that blasting operations to clear coral reefs around Pacific islands sometimes produced a large wave, raising the possibility of creating a "tsunami bomb".

Explosive tests carried out in waters north of Auckland led scientists to conclude that the weapon was feasible and a series of 10 massive blasts offshore could create a 10-metre (33-foot) tsunami capable of inundating a small coastal city.

"It was absolutely astonishing," Waru said.

"First that anyone would come up with the idea of developing a weapon of mass destruction based on a tsunami... and also that New Zealand seems to have successfully developed it to the degree that it might have worked.

"I only came across it because they were still vetting the report, so there it was sitting on somebody's desk (in the archives)."

Waru said the project was shelved in early 1945, despite the success of initial, small-scale tests.

"If you put it in a James Bond movie it would be viewed as fantasy but it was a real thing," he said.

Among the other oddities in the archives are Defence Department records of hundreds of UFO sightings by members of the public, military personnel and commercial pilots, mostly involving moving lights in the sky.

Some of the accounts include drawings of flying saucers, descriptions of aliens wearing "pharaoh masks" and alleged examples of extra-terrestrial writing.

New Zealand's most famous close encounter was when a television crew recorded strange lights off the South Island town of Kaikoura in 1978.

However, in a disappointment for ET spotters, the military concluded the lights could be explained by natural phenomena such as lights from boats being reflected off clouds or an unusual view of the planet Venus.

Waru said it was seemingly humdrum documents, like school magazines from the early 1900s extolling the virtues of the British Empire, that provided a window into the attitudes of the past.

"There's masses of records and kilometres of important files but you realise pretty quickly that every piece of paper is related to an individual at some point in time," he said.

"So it gives the modern researcher a peek into the private lives of individuals, which I found interesting -- divorce files from Dunedin, letters a young soldier wrote home to their parents."