The ash was still wet under their feet when an adult, a child and a dog stood on a beach overlooking the newly formed Rangitoto Island that had spectacularly erupted in front of them.

Their footprints, laid in the soft fallout from Rangitoto about 650 years ago, hardened as the ash cooled to form a concrete-like layer over a plateau on Motutapu Island.

The footprints lay hidden for centuries until the 30cm thick slab of ash was exposed by coastal erosion and discovered by Department of Conservation workers.

The slab, discovered a decade ago, was removed for storage on the island for protection.

Yesterday the block, which weighs about 250kg, was blessed by Maori at a special ceremony on Motutapu before being lent to the Auckland War Memorial Museum.

It will occupy a special place in a major exhibition on the volcanoes of Auckland at the museum.

Rangitoto is believed to have exploded into existence in the 14th century, with at least four major ash showers and several minor ones.

Neighbouring Motutapu was used for gardening, fishing and manufacturing stone tools.

DoC Auckland area manager Beau Fraser said it was unclear how many people lived on Motutapu but it had been a major population centre.

Mr Fraser said the ash deposit where the footprints were found was fraying at its edges as the sea eroded the ancient beach underneath.

The deposit was about 1m deep and footprints had been found over about half a hectare.

DoC archaeologist Andy Dodd said footprints in the area were first discovered in 1981.

There was probably a hasty exit from the site, although there was no evidence of fatalities.

"There would have been fizzing and hissing with hot ash rising, cooling and then coming down in showers."

Mr Dodd said the site made for interesting excavations as below the ash layer there was evidence of bird bones, such as those of the extinct native crow and moa.

Rangitoto is the youngest of Auckland's about 50 basalt volcanoes and the only one to form during human habitation. The Domain and Mt Albert are more than 100,000 years old.

Emily Karaka, of the Ngai Tai ki Tamaki iwi, said she was descended from the Maori of Motutapu Island.

Her great-great-grandmother had lived at Home Bay in the early 1900s.

"I first came to this site on a school trip 22 years ago. I didn't know the history but I fell on my knees - there was some presence and I felt very moved."