"There will now be no such venue left for users of the park between Napier and Palmerston North. Despite pleas from the [RUG] to keep the base going, they chose to ignore these and proceed with their pre-determined intention to close the base," he said.
"It's conclusive evidence of the Government's continuing programme of 'hollowing out' the department for the so-called sake of 'efficiency', a euphemism for cost-cutting and reduction of nuts-and-bolts conservation which New Zealand so desperately needs - now more than ever."
The base previously housed eight staff working in reserves which are now managed by DOC's Hawkes Bay, Wairarapa and Manawatu Districts.
The remaining Hawkes Bay-focused staff relocated to Napier more than two years ago.
The department said since then, the Manawatu-focused staff positions were the only ones at the base.
Elderkamp said one staff member was was never replaced after leaving in 2013, and since April this year there has only been one field ranger permanently stationed at the base.
"One of the reasons given for its closure was that it's under-utilised - the only reason for that is DoC's deliberate strategy to cut field staff and operations from the base leading to the inevitable, albeit erroneous, conclusion to close it."
Elderkamp said what he and and many others feared a few years ago, when DoC appealed for greater community involvement in conservation work, had "come to pass".
"Effectively, community groups and volunteers have taken over a lot of DoC's responsibilities across the country, at the cost of DoC jobs."
Elderkamp said the group first learned of the intended closure via social media, which highlighted the department's "cut-by-stealth" approach.
"At present we are still awaiting DoC's evidence in support of the intended closure," he said.
John Montgomerie from the Heretaunga Tramping Club, was concerned at the loss of more experienced Doc staff and their local knowledge.
"The intellectual property loss here is massive. I really feel that all this will have a negative impact on the Ruahines and a serious effect on our trust and faith in DOC from here on," he said.
Palmerston North's Anthony Behrens, who sits on the executive of the Federated Mountain Clubs of New Zealand, agreed.
"The loss of a very experienced and respected staff member who had good working relationships with locals has got to be a backward step for infrastructure and conservation in the area."
He accused the department of being more interested in keeping costs down than conservation, and focusing on tourist areas.
"While new tracks are being proposed on the Central Plateau, track maintenance is about to be halved in the Ruahines. The loss of the Onga Onga base seems to be further proof that DOC's priorities are shifting away from New Zealand's heartland and onto places where we can make a buck," said Behrens.
His group had lodged a request under the Official Information Act to be able to assess the reasons for the closure.
Behrens is also member of the Ruahine Whio Protectors and the Palmy Pine Pulling Posse.
"I and dozens of others put thousands of hours a year into helping in the vulnerable Ruahine Ranges and the wildlife that sometimes struggles to live there - many of us see this decision as an insult. We weren't consulted with until we found out about what seemed to be a fait accompli.
DOC's Manawatu Operations Manager Allanah Irvine said issues raised by the RUG members had been taken into account as part of the Department's decision-making process.
"You can be assured that these concerns were well considered and debated within the Department prior to any decision being made. The decision to close the base was not taken lightly."
Work in Central and Southern Hawkes Bay districts, including all work in local reserves near Onga Onga, was delivered by the Hawke's Bay Operations District, based in Napier.
"Conservation work on public conservation land in the Hawkes Bay will continue to be provided by DOC's Napier based team, as it has for a number of years."