Researchers are calling for the protection of round boulders on an Auckland hilltop that they say are of international historical importance and were placed there by an ancient pre-Maori society.

The site is being gouged by earthmoving machinery for a new hospital at Silverdale, next to the Orewa-Whangaparaoa turnoff.

Martin Doutre, author of the book, Ancient Celtic New Zealand, and fellow researcher Russell Ireland say that only two large boulders remain out of a dozen. One has a girth of 3m. Many smaller ones have been smashed, they say.

The cluster of boulders was uncovered 38 years ago when roadmakers cut into the crown of the scrub-covered hill.

"It sparked a lot of mystery over how they got there," said Mr Doutre. "They were concretion boulders, which can only form in sea sediments, yet they had made it to the top of this high, yellow clay hill."

A bulldozer dug around them and shunted them down and out of the way.

Four of the smaller ones were lifted in a sling and trucked off by a contractor for the Auckland City Council.

They were placed at the Stanley St entrance to the Auckland Domain. Another is in the University of Auckland's geological display.

Mr Doutre said the boulders were artefacts deserving of protection.

Some boulders showed ancient etchings of geometric designs similar to those on structures in Britain dating back to 3150BC.

He believed they were placed on Silverdale Hill as one of many structures built for calendar and surveying functions by fair-skinned people known as "Patu paiarehe" - before the Maori came from Polynesia about 800 years ago.

Mr Doutre said the stones were placed exactly on a line of mound and cairn structures stretching for more than 70km through the Auckland isthmus - from Mt William in the Bombay Hills to Moir's Hill north of Puhoi.

Such round structures were used in Costa Rica, Mexico and Bosnia in central Europe.

Mr Ireland said the two large boulders had been rolled down the hill by a digger and were badly marked.

He had asked Rodney District Council a month ago to protect them.

Council spokesman Mike Isle said the boulders did not have heritage protection under the District Plan.

"However, the council planner believed they have value to the immediate area and are an icon known with a lot of affection.

"Part of the resource consent for the new hospital is that the two large boulders will be part of its landscaping and placed as close as practicable to their original position."

Mr Isle said early pictures showed the smaller boulders already damaged.

Geological Society spokesman Bruce Hayward said there was no mystery how the boulders got on thehill.

He said they were 70 million years old and pushed up from the sea floor and the enclosing countryside eroded over time, leaving them exposed.