Officials should not have withheld a key document behind the government's decision to proceed with a "divisive" LNG terminal, the chief ombudsman has ruled.

The document - released in full following the ruling - concluded access to liquefied natural gas (LNG) was unlikely to affect average electricity prices and there were other ways to improve New Zealand's energy security.

The climate advocacy group that laid the complaint said the lack of transparency over such an expensive and contentious project was concerning.

The document summarises the findings of Concept Consulting, which the Ministry for Business, Innovation and Employment (MBIE) commissioned to model the effect of building the LNG facility.

The ministry has downplayed the consultants' conclusions, saying the set of slides was a "draft of an informal working document" and therefore officials did not correct anything they disagreed with.

However, Chief Ombudsman John Allen found the document provided "important context and limitations" for the advice and modelling.

He found the reasons for withholding it were not valid, and even if they were, there was "very high" public interest in releasing it, especially in an election year.

"The government's plans for an LNG import facility in Taranaki has proven divisive," he wrote.

"There is also accountably in the expenditure of public funds, and I note that the plan is estimated to cost upwards of $1 billion."

Rather than a rough draft, the document was "fairly advanced", Allen said in his decision.

"MBIE had already engaged in back and forth with Concept and had the chance to improve on and refine its earlier drafts."

LNG will not help with electricity prices most years - document

The government confirmed in June it was proceeding with plans for a facility in Taranaki, saying importing liquefied natural gas was the best option to see off a "dry year" risk, when hydro lake levels are low, while more renewable energy was developed.

The proposal has been widely questioned, including by some gentailers, and has faced local opposition in New Plymouth.

A group of 11 climate and consumer advocacy organisations wrote to the Auditor-General earlier this week, urging his office to open an inquiry into the procurement process.

RNZ has previously reported on a partially-unredacted version of the consultants' document, which revealed there was "low need" for LNG imports.

The latest version includes a summary slide, labelled 'Key insights', which concluded that access to LNG "shouldn't materially affect average electricity prices".

LNG access "should" reduce prices in the most extreme dry years, the document said.

However, that was only if renewables did not keep up with demand growth, there was further decline in domestic gas supplies - beyond what was projected - or there was a "black swan" event where all of the coal-burning units at Huntly Power Station were out of action.

"Other resources can provide a similar system security service to LNG", the document concluded.

The alternatives it had modelled included coal at Huntly, extra underground gas storage - such as one proposed at Tariki gas field - and building even more geothermal, wind and solar generation capacity.

The government has said that having an LNG facility would save households $265 million a year, because the security it will provide would help to cut forward prices on the energy market.

The Electricity Authority - among others - has attributed the fall in prices to a glut of renewables coming online, with more planned for the near future.

Lawyers for Climate Action spokesperson Laura MacKay, whose organisation complained to the ombudsman, said a promised reduction in electricity prices had been "a key justification" for LNG.

The new information undermined that argument, she said.

"The absolute key point for me is why have these particular conclusions been withheld?

"It's confusing, and I do honestly find the lack of transparency really concerning."

Her organisation had first asked for a full version of the document in February,

"If LNG is such a good idea, why has it been so difficult to access this kind of relevant and important information?"

MBIE withheld gas supply data

Both MBIE and the government have criticised Concept Consulting's modelling since the first set of redactions were lifted in June, saying it relied on "optimistic" assumptions about domestic gas supply.

The latest version of the document shows the consultants considered their assumptions, which used MBIE's own low-end projections, were "conservative".

Emails previously released to RNZ show ministry officials worked closely with Concept Consulting to develop the scenarios the model tested.

However, a letter from MBIE accompanying the newly-released document reveals the ministry did not share the latest gas supply data it had with the agency.

"That data showed that gas reserves had dropped by 23 percent relative to the previous year's estimates," energy markets policy director Rebecca Heerdegen wrote.

"At the time, rather than revealing commercially sensitive information, we did not correct or engage with Concept's opinion that the gas assumptions are conservative."

Because the work the ministry had commissioned was the modelling outputs themselves, "we did not seek to correct Concept on opinions they expressed in the slide pack where we disagreed or held additional information that Concept was not privy to," she wrote.

Concept has since independently completed further modelling, with the results drawing similar conclusions to its original report.

In an additional report it created for the four electricity gentailers last month, it repeated its earlier findings that many of the factors that led to high energy prices in 2024 no longer existed.

"The dry-year problem is not getting worse - indeed the overbuild of renewables is reducing the need for flexibility resources from thermals or demand response."

RNZ has requested comment from Energy Minister Simeon Brown, including whether the government is still planning to sign contracts for the LNG facility prior to the election.

His office acknowledged the request but was unable to provide a response prior to publication.