A large group of Moa bones have been found in South Canterbury.

Contractors discovered dozens of bones from what may be the tallest bird species recorded in what has been called an increasingly rare find.

The bones of a female South Island giant moa were found with at least one bone from a much smaller male at a still-secret location in rural South Canterbury.

They were found in spoil removed from a small, deep trench dug in what was once an ancient swamp.

John Bisset Alpine Energy network design and project manager David Sutton and South Canterbury Museum director Philip Howe with moa bones discovered by contractors in rural South Canterbury.

A mix of 40 bones and bone fragments - leg bones, vertebrae, a pelvis and ribs - were pulled from the soil by increasingly surprised contractors this week.

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"They were stoked when they realised what they were dealing with," Alpine Energy network design and project manager Dave Sutton said on Friday.

"It's not every day you dig a hole and find a moa. Only one small hole and this is the result."

The crew from Ashburton-based contractor Lemacon stopped work when they saw the dark swamp timber poking from the soil was not as it appeared.

Sutton arrived as workers assessed the assemblage of oversized bones. The 400mm wide, 2m long, 2m deep trench had yielded "something pretty special".

It appeared the cut had gone through the remains of at least one animal that looked to have fallen on its side after becoming stuck in the long-gone swamp.

South Canterbury Museum was contacted and the "not insubstantial" muddy bones were examined by director Philip Howe.

"On inspection it appears these are the bones of at least two individuals, a very large female and a much smaller male," Howe said as he examined the bones at the museum on Friday.

"Which begs the question: was this the tragic outcome of a Sunday picnic at the swamp with the moa family?"

Female South Island giant moa stood about 2m high at their back and could reach foliage more than 3.5m off the ground. They were the taller of what was the tallest known bird species.

If the South Canterbury female leg bones were from the same animal, there was a chance its pelvis could have been at least 1.6m from the ground.

It would be virtually impossible to estimate the age of the bones without using expensive carbon dating or an exhaustive site examination, Howe said.

The trench was filled but Sutton confirmed its GPS coordinates had been recorded and that it could be re-excavated for further work.

Howe was keen to record as much detail as possible about the site. He hoped to check for other bones but there would be no full-scale excavation.

Howe reckoned it was likely the area was once surrounded by forest cut by well-trodden pathways worn through by wandering, foraging moa.

It may be that the South Canterbury birds now in pieces on the museum floor were dashing through the bush when they "blundered into" the swamp.

South Canterbury was rich with moa. The remains of five of nine known moa species have previously been found at sites around the province, he said.

The latest find would be cleaned and stored at the museum, along with any other data the museum could glean from the recently-discovered site.

"In this day-and-age we are not finding moa as much as we used to, it is rare that someone stumbles-upon moa bones while digging for something else," Howe said.

"This will be a very useful, valuable collection that may, one day, provide further useful information to someone researching moa and, in particular, that site."

Howe commended Alpine Energy and its contractor for how they responded to the find.

He did not encourage people to hunt for and excavate moa remains themselves and urged anyone who found moa bones to contact the museum or experienced university-based researchers.