'He's our best friend ... we aren't gonna stop, no matter what': couple's costly rescue bid for Jack Russell dog trapped down West Coast tomo

A West Coast couple are spending $50 an hour hiring a digger in a more than 24-hour effort to rescue their "best friend", a Jack Russell dog trapped down a tomo.

Three-year-old Jacko was possum hunting with his owner Shane Curtis, a professional pest controller, near the Inangahua yesterday morning when he chased a possum into the tomo.

Tomo are sinkholes formed when ground water dissolves underlying limestone rock.

The tomo where Jacko fell. Photo / Supplied

Curtis' partner Willow Leigh said the narrow tomo Jacko entered concealed an at least five metre drop, and the dog has been trapped inside since just after 9am yesterday.

The couple have not stopped trying to rescue him since, starting with shovels and continuing with a hired digger since about 8am last night.

Joined by Curtis' father and brother, they haven't slept at all, Leigh said.

"He's our best friend ... he even cuddles up with my cat. He always goes hunting with my partner, he loves catching possums, he always comes home with a big smile on his face."

Shane Curtis and Jacko, pictured before Jacko became trapped in the tomo. Photo / Supplied

Both had taken time off work Jacko's rescue had already cost them almost $1000 in digger fees.

But he was worth every cent, Leigh said.

"We don't care about money. We just care about getting this guy out alive ... we aren't gonna stop, no matter what, rain or whatever. We've been up all night and we are not stopping, the poor little dude."

A digger is being used to try and rescue Jacko. Photo / Supplied

The couple did not know if Jacko was injured, but based on his yelping believed he might be.

They had managed to opened up enough ground to toss some dog roll to him, she said.

"We gave him dog roll as it was the easiest thing to get down there and we have a roast lamb cutlet for him when he gets out."

They had tried to call police, fire and cave rescue teams for help, but had not been able to get through on landlines.

The shaft of the tomo where Jacko is trapped. Photo / Supplied

Any help that anyone could offer would be appreciated, Leigh said.

Jacko is trapped in a rural area, which she described as being where the powerlines cross the river near the State Highway 6 route to Westport, about five minutes drive from Inangahua.

Anyone who can help can call Leigh on 027 366 9761.