A fossil of a giant crab thought to be millions of years old is being exposed by Masterton man Dennis Johnson, who is slowly chipping away at the rock in which it is embedded.

Mr Johnson spotted the crab claws in a rock lying in the garden of his partner's son's house in New Plymouth.

"I saw this rock in his garden with its claws sticking out and I was like 'wow'. I said if you ever move away from here please don't leave the rock, take it with you or let me know.

"He said, 'oh well, take it'. I grabbed it with great haste and threw it in the back of my wagon."

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Mr Johnson said he had taken the rock to GNS Science in Wellington who said the fossil was somewhere between five and seven million years old.

"They can't actually date it with any accuracy because they don't know where it's come from. That's one of the ways they date things, by the location they come from, because they know the history of the area."

Mr Johnson, a retired fireman, started chipping into the rock a few weeks ago and said he had spent about six hours so far cutting into the hard stone using an angle grinder.

"God only knows how much longer. I'm not an expert on it but it's obviously going to be a considerable amount of time. It's very slow work."

He said his first mission was to cut off the bulk of the rock, and then he would start working towards the body of the crab, but using different tools and techniques.

The rock has been supported with a strap because there is a large crack to the right of the claws. Mr Johnson has been working on the fossil at Henley Men's Shed.

He took up sculpting as a hobby 10 years ago, mainly working with Oamaru stone. He said exposing the giant crab was like no other project he had attempted before.

"It's pretty awesome really -- flabbergasting really when you think of it," he said. "GNS said this crab is bigger than any they have got in their collection, judging by its claws."