A stagnant effort to save the Hauraki Gulf - now to be salvaged with a Government intervention - could have been done better, an official inquiry has found.

The much-vaunted Sea Change Plan, developed over four years by a cross-sector group, aimed to create a marine spatial plan that would recommend what activities should take place and where in the 1.2 million hectare gulf marine park.

Bringing together 181 separate proposals, it would determine what areas should be safe-guarded and shape plans to come, at a time Auckland's blue backyard was under unprecedented pressure from over-fishing and pollution.

But, this year, the Hauraki Gulf Forum warned the effort would "probably fail" because it wasn't enforceable and was "probably unfundable, at least under present arrangements", given the cost involved in implementing it.

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The forum leaders were especially scathing of Crown representatives appointed by the Ministers of Conservation, Fisheries and Maori Affairs, who, rather than working together to solve problems, had been "passive reporters of ministry actions" or "advocates for single issue policies".

Conservation Minister Eugenie Sage and Fisheries Minister Stuart Nash have since moved to revive Sea Change by establishing a new ministerial advisory committee to drive its implementation.

Meanwhile, a new report by Deputy Auditor-General Greg Schollum has highlighted where the plan had succeeded and failed.

Schollum described it as a large and ambitious undertaking, addressing a complex problem that involved many overlapping interests.

The project had required "significant commitment" from central and local government, as well as from the various interested parties on its steering group.

In many ways, he said, it had proven a successful example of a collaborative approach as it resulted in a completed plan with general support from those who prepared it.

Yet it hadn't been an easy one for agencies to implement, leaving those involved frustrated at a lack of progress.

Schollum found the agencies – which included the Auckland Council, Waikato Regional Council, the Ministry for Primary Industries and the Department of Conservation – weren't as involved in developing the marine spatial plan as they could have been.

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"There needed to be a balance between giving the stakeholder-led collaborative group enough independence while still having the right amount of involvement from the agencies," Schollum found.

"There needed to be more communication and discussion of the plan with stakeholders as it neared completion.

"Because there was not as much engagement as there could have been, not all of the stakeholder groups agreed with the final plan."

Towards the end of the project, and when the plan had been finished, there was little discussion with the community about it.

The plan could have benefited from wider communication and, from that, gained wider support from the community.

"When the project was set up, certain matters, such as setting an appropriate scope, needed to be considered so the central and local government agencies could easily implement the plan," Schollum found.

"The agencies could also have prepared for how they would implement the plan, including how they would work together with other organisations and stakeholders, and what the role of mana whenua would be."

Schollum urged all agencies setting up collaborative planning projects to look to Sea Change as a lesson.

"This project's success will ultimately depend on how the marine spatial plan is used and whether its recommendations are incorporated into local and central government's decision-making," he said.

"It is important for the agencies to consider how they will work together to progress implementing the plan and get support from affected stakeholder groups, such as commercial and recreational fishing groups.

"There is a risk that if there is no further consideration of the recommendations in the marine spatial plan, the money and effort spent on the project will largely be wasted."

Estimates suggest the gulf today supports less than 45 per cent of the fish "biomass" that it did in 1925, after decades of over-harvesting.

Snapper and rock lobster populations were well below target stock levels, while numbers of john dory, gurnard and trevally had also fallen to worrying levels.

Fishing had cut snapper rates by 80 per cent, with the biggest impact on old, large fish, and growth rates had slowed.

Similarly, crayfish numbers have been slashed to about 20 per cent of their 1945 levels.

While the gulf was subject to the Hauraki Gulf Marine Park Act, the Crown had taken only a limited account of the legislation after courts ruled it had conflicting objectives.

Beyond this act, just 0.3 per cent of the gulf was protected by statutes.

There were six marine reserves constituted under the 1971 Marine Reserves Act, yet only one had been created in this century, and since the marine park act had been in place.

With Auckland's population having boomed over the past decade, increasing by almost a quarter of a million people between 2006 and 2016, calls were growing for new marine protected areas legislation.