Monitoring the crater lake helps scientists to calculate the volcanic alert level of Ruapehu.

The Ruapehu skifield company has stopped paying for the eruption detection system on the active volcano, raising questions over its future.

Under an agreement with the Department of Conservation (DOC), Ruapehu Alpine Lifts (RAL) has been paying about 80 per cent of the approximate $62,000 annual operating cost of the system since significant upgrades began in 2010. DOC and the community have split the remainder.

But documents obtained under the Official Information Act show RAL has not paid invoices for the Eruption Detection System (EDS) since April.

RAL says the money hasn't been paid because its contribution is still under negotiation.

Sources say the uncertainty around funding threatens the system's future as it undermines its importance and other community funders might pull out.

The company disputes that, saying on Wednesday: "While funding sources and the amounts each contributor pays may be under discussion the ongoing existence and operation of EDS isn't under debate."

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RAL recorded a $4m loss on earnings of $28m in 2018. It is in the middle of a $100m infrastructure upgrade that includes a $25m gondola to open in 2019.

DOC believes the company, which is a non-profit and tax-exempt, has a moral and legal obligation to pay the lion's share of the EDS costs because its staff and customers are the primary beneficiaries.

It says there wouldn't be an eruption warning system at Whakapapa if there was no skifield.

A skifield worker snow boards at Turoa while Mount Ruapehu erupts in the background in 1996.

But there is no contract in place and RAL argues that the payments are voluntary, sources say. It believes central or local government should pay, as they do for earthquake and tsunami monitoring.

The EDS is part of DOC's Volcanic Alarm Network in the Tongariro National Park.

Data from GeoNet is designed to trigger sirens and automated voice messages on Whakapapa skifield in the event of an eruption, as well as sending messages to skifield staff and police.

Mt Ruapehu is one of the most active volcanoes in the world to have ski areas - eruptions in 1969, 1975, 1995 and 2007 sent lahars into the Whakapapa field.

Under previous chief executive Dave Mazey, RAL agreed to pay for 80 per cent of the cost of upgrading the system - including a one-off payment of $17,000 plus $70,000 of upgrade costs spread over 10 annual payments.

It also agreed to cover 80 per cent of the community's share of ongoing operatings costs.

But new chief executive Ross Copland has questioned the arrangement.

Official Information Act documents include an email from Copland a year ago pointing out that RAL had been hit with "very significant increases" in the licence fees it paid DOC for its skifield concession.

This was due to the "aggressive repricing negotiations" being driven by DOC head office, he wrote.

"I'm afraid it's looking less likely that we'll be in a position to contribute financially to the [EDS] system in the future," Copland wrote.

In June 2018, DOC wrote to RAL offering to reduce the company's share of the operating cost from 80 to 55 per cent, with DOC's share increasing to 35 per cent.

Under this proposal, RAL would save on average about $15,500 a year.

Copland wrote back describing the EDS as "public safety 'Civil Defence' infrastructure".

He wanted to know if KiwiRail was required to pay for a lahar warning system on the eastern side of the mountain to protect rail passengers.

(KiwiRail, along with Transpower, the NZ Army, Genesis Energy and Ruapehu District Council provide funding to maintain that system.)

STUFF Mt Ruapehu erupts in 1995.

Copland told DOC the company was happy for the equipment to remain in RAL buildings and mounted on its lift towers and would continue to provide "free lift access, snowmobile and groomer transport" for technicians carrying out maintenance.

Other parts of Copland's correspondence were redacted.

Dr Harry Keys, DOC's technical advisor for volcanology in Tongariro, told Stuff negotiations over RAL's share of the cost were ongoing.

He was concerned it was putting a "cloud" over the EDS and sending a message that it wasn't important.

"This is taking a whole lot of extra time and time is limited. While we're doing this stuff we're not doing something else," Keys said.

"I'm concerned that this whole process takes resources away from managing volcanic risk."

He could understand a company would want to reduce costs, but from a health and safety point of view alone the EDS was vital.

In 2007, one of RAL's groomer drivers had a narrow escape when he had just seconds to get out of the way of a lahar near the Far West T-Bar.

"That's one clear demonstration of the importance through health and safety of staff - it's an RAL responsibility."

Keys said Ruapehu was particularly hazardous because about half of its lahar-generating eruptions occurred without warning.

"Sending a lahar down the skifield, which might have hundreds of people in the way - you need a system to warn people and staff."

He hoped DOC's gesture of reducing RAL's share of operating costs to 55 per cent would help settle the issue, as it was a fairer split.

Alexander Turnbull Library The mountain erupts in 1945 as trampers look on.

Copland said in a statement that the EDS was an important tool for managing risk to park visitors and RAL remained "fully supportive of the ... system managed by DOC, we have communicated this to the department on a number of occasions".

He pointed out that RAL was one of 67 licensed concessionaires operating within the Whakapapa and Turoa ski areas who had operations reliant on shared infrastructure provided by DOC.

"We have recently renewed the licences ... so we are working through the respective responsibilities for RAL and [DOC] across a range of shared infrastructure, including the EDS, to ensure there is a sustainable plan in place for maintenance and renewal over the next 60 years ... and an equitable allocation of the costs."

Copland said the National Park was rapidly changing from being dominated by winter skiers to a much broader range of visitors year-round.

"This in itself is exciting but also presents a challenge for us all moving forward requiring careful planning, implementation and working closely with stakeholders."