The New Zealand Defence Force is investigating the potential cost of moving Devonport Naval Base.

Documents released by the New Zealand Defence Force shed light on a review into the future of the base.

The documents show Picton has previously been mooted as a possible replacement.

But NZDF staff are refusing to be drawn on whether any future plans could include relocating vessels and personnel to a new Southern Ocean base in Dunedin.

It was reported last week that the Royal New Zealand Navy was rumoured to be considering Dunedin as a possible regional base for new ice-strengthened vessels due in the next few years.

NZDF initially insisted there was no plan to expand the Navy's footprint in Dunedin, but would not comment when asked if such an option was being considered.

However, documents on the future of Auckland's Devonport Naval Base, released to the Otago Daily Times on Friday, showed alternatives had been considered in the past, and were again under the microscope.

A 2013 document, the Defence Estate Road Map 2050, had outlined five options for the future of the Devonport base, ranging from consolidation on the Devonport site to a complete shift of the base to Picton.

Option five, which ranked second highest in a comparative analysis, suggested a mixed approach - consolidating the Devonport base, where all operational units of the Royal New Zealand Navy are currently based, but also establishing a new forward operating base elsewhere in New Zealand.

The option envisaged new facilities built away from Devonport, at an estimated cost of $180 million, catering for up to four vessels.

The paper recommended Devonport's retention and consolidation, but Cabinet later directed the NZDF to remain in current locations.

A briefing for new Defence Minister Ron Mark, prepared in December last year, said the
2013 report had been "superseded'' by the Defence White Paper 2016.

The paper outlined plans for new investment, including in new ice-strengthened ships, and was said to signal a greater emphasis on Southern Ocean operations.

The paper said Auckland remained the best location for the Navy's main operating base, but if the operating model changed, "the conclusions of this analysis will need to be revisited''.

One of the factors to consider was listed as "proximity to activity'', the briefing noted.

"Is the Navy's home port or Naval units located close to where force elements undertake their activity or further away?''

Another issue was the coalition agreement between Labour and New Zealand First, which required a feasibility study on options for moving Ports of Auckland, the briefing said.

That, in turn, had "implications'' for the future of the Devonport Naval Base, which relied in part on support from Ports of Auckland for bunkering fuel, tug boats and other services, it said.

As a result, a fresh NZDF study was being undertaken in the first half of this year "on the costs of moving Devonport Naval Base''.

NZDF staff did not respond when asked on Friday if the option of a Navy forward operating base would be considered again as part of the 2018 study.