Photo: RNZ / Tracy Neal

Time and tide - and a few massive earthquakes - have opened a portal into New Zealand's distant past.

Scientists said the discovery this week of an ancient footprint in Golden Bay rock, is further proof that dinosaurs once walked, ate and swam in the area about 70 million years ago.

They said the trace fossil - uncovered during a high school field trip - was a fantastic scientific discovery, and added to almost 50 others found in the last decade.

Students from Collingwood Area School this week helped GNS scientists sweep away estuary mud to discover another chapter imprinted in the rock extending from a small outcrop near a remote Golden Bay swamp.

Student Violet Wells said it was an honour being among the first to lay eyes on something that had been hidden for tens of millions of years. She could not imagine that her backyard was once a feeding ground for animals larger than an elephant.

"It's quite amazing that we're where they actually walked - this is an actual footprint, it's not just something that someone's made. It's just hard to imagine really."

Greg Browne points to the new footprint Away on a dinosaur discovery Class shot Dr Greg Browne explains the history A shape in the rocks was not a print this time. Dr Greg Browne brushing for clues in the rock wall Marianna Terezow Sweeping and washing down rocks Lucia and Marianna Whanganui Inlet

GNS sedimentologist Greg Browne discovered the first footprints by accident about 15 years ago in an area deep within the Whanganui Inlet. The large tidal estuary just south of Farewell Spit opened out on to the West Coast and the often stormy Tasman Sea.

Dr Browne said the area was linked to the Taranaki basin and geologists had been studying its rocks in line with oil and gas research.

"When I first came here we were looking at the geology for that purpose. I saw these structures and I didn't know what they were and I would try and explain them in some sort of physical process, like it was a current or something like that which was forming them."

He concluded that was not possible. Years of research, analysis and peer reviews pointed to them being dinosaur footprints.

Dr Browne said the discovery ranked alongside the dinosaur bone found in Hawke's Bay by amateur palaeontologist, the late Joan Wiffen, and that the footprints found in North West Nelson were extremely rare.

"This is the only locality in New Zealand where we have dinosaur footprints, and the only locality in the whole South Island where we have any dinosaur... anything - bones or footprints, so it's a very unique site," he said.

Dr Browne described the prints as circular, like that of an elephant or hippopotamus. Some are larger than a car's steering wheel, but tell the story of a thoroughfare and feeding ground for the 100-tonne seagoing sauropod.

GNS head of palaeontology Lucia Roncaglia said their story has emerged through aeons of tectonic movement. She said all the layers of the environment they lived in became submerged under a very deep sea, millions of years after their time. The process cemented their prints in the rock.

"After many millions of years all of this material came up again, through tectonism primarily, and that's why it is exposed and why it happens to be in the same kind of environment," Dr Roncaglia said.

Dr Browne said the world map was quite different 70 million years ago. The dinosaurs walked across Australia and Antarctica and drifted with the earth's shifting plates.

"We were part of a much bigger continent called Gondwana. It consisted of Australia, Antarctica as big plates and New Zealand was a much smaller plate which we now call Zealandia, and we were starting to split away from the other continental blocks.

Dr Roncaglia, an Italian-born micro-palaeontology specialist, said the discoveries were important not only because they were so far from the rest of the world, but because they also fed into climate change research.

"We work very much with palaeo-climate research, and we are the only ones who can put the point of difference and provide this data to the models that get done in the Northern Hemisphere but that are global models."

Dr Roncaglia said New Zealand in the time of dinosaurs was much closer to the south pole at about 70 degrees south in Latitude, and in a much warmer world than today.