The estimated construction cost for a new hut along the Milford Track in Fiordland has nearly doubled to more than $3 million since work started on the initial design.

The 40-bed Mintaro Hut will replace the existing one which sits in a potential rockfall zone.

The work was was initially expected to be finished for last summer's walking season.

The hut is nestled amongst the trees almost hidden from view and so far, the framing is up and there is a deck but no walls or roof yet.

Builder John Henderson said: "We've sort of seen a bit of progress now at last. It's taken a long time to get out of the ground. It's been a very wet spring, had lots of rain. We pulled out one Sunday and by the following Thursday there'd been a metre of rain on site so probably just as well we weren't here".

His crew of usually seven or eight started work pre-Covid-19, but they were locked out and then winter took hold.

"I've done quite a few huts but this one's a bit different. It's got big portals in the main rooms so you've got a big clear span, lot of tanalised timber everywhere, very well insulated this one and there's been a real focus on the airflow in the rooms to keep the rooms dry. Hopefully it will all come together and work out well," Henderson said.

Logistics are a big challenge.

"When the weather's right, we use the barge on the lake and two helicopters to get it emptied. We've flown stuff from Knob's Flat, we've had some stuff from Milford and it all depends on the weather like if it's cloudy, you can't get over the passes.

"You can pretty well come up the lake in a barge every day but then sometimes you can't even get here because of the mist and the fog."

Official documents from 2018 show the initial price tag was $1.8 million, which included a 15 percent contingency, design, planning and the build.

Department of Conservation principal ranger Grant Tremain said the new $3.4 million estimate reflected the remoteness of the site and the increasing costs.

"The true price is when you get half a dozen or 10 different people to price the construction and work, and that's where that went back up - the extra labour costs, the transport costs and the component into here. That's a really hard calculation to make - how much would it cost to fly everything up or barge it? How do you do it?"

He acknowledged the construction delays, saying it had taken more time to manage the project and to assess the site's risks.

"There's a really testing environment up here. This would be a substantial build in town, let alone in the middle in the heart of Fiordland in what is really tough, steep environment and topography. Yeah, to get to here it does show it can be done, so [it's] pretty fantastic to see it," Tremain said.

Mintaro Hut is about two kilometres away from the old one.

The current hut is more than 30 years old and advice from GNS about rockfall meant it needed to be closed and the new one built.

Tremain said the new build was far superior and out of the way of known rockfalls.

It also has a specially engineered septic tank that should only need emptying every two to three years instead of annually.

The hut will be warmer too - it gets more sun and has double glazing.

Destination Fiordland general manager Madeleine Peacock said being able to stay in a quality hut would encourage more to consider walking the Milford Track.

"People want to be able to be semi-comfortable when they walk these tracks. You know a day of hard tramping in the rain, it's nice to know that at the end of it, you're going to come to somewhere that's warm, somewhere that's safe, where you can rest and recover for the next day and dry your boots and a few home comforts," Peacock said.

The new Mintaro hut is on track to be open by March next year.