Back in April it emerged the government had ordered a review of the way the Reserve Bank operates. Delays releasing the report has fueled talk that the scope of the review may have been wider than initially suggested.

A review of the Reserve Bank may have recommended far more wide ranging changes than even Finance Minister Steven Joyce expected.

Back in April it emerged that after years defending the Reserve Bank's operating model, Joyce instructed Treasury to commission a review of aspects of its structure.

At the time it appeared the review, conducted by former State Services Commissioner Iain Rennie was to be limited only to the Reserve Bank's process for determining movements in the official cash rate (OCR), and whether the bank should be in charge of the legislation it operates under.

MAARTEN HOLL/STUFF The entrance to the New Zealand Treasury, at 1 The Terrace, Wellington, immediately opposite the Reserve Bank.

But there is growing speculation that Treasury may have given Rennie a wider brief than Joyce suggested. Treasury has refused to release the terms of reference of the report, or the final report, which was delivered back in April.

READ MORE: Reserve Bank governor's unique power over interest rates to come under review

Cameron Bagrie, chief economist for ANZ, said without the terms of reference he was "flying a bit blind", but it was possible the review was headed towards recommending a model used across the Tasman, where powers are split between the Reserve Bank of Australia and the Australian Prudential Regulatory Authority (APRA).

"The consensus seems to be that the review is about monetary policy," Bagrie said.

"I suspect it's broader and maybe they are looking at whether we have an Australian model where they have the RBA for monetary policy, financial stability, markets, payments et al and APRA for the prudential/regulatory side."

Splitting banking regulation from the Reserve Bank would reduce the scope - and power - of the central bank considerably.

Top officials within the Reserve Bank are said to believe Rennie's report is something of a power grab by Treasury.

Michael Reddell, the former special advisor to the Reserve Bank, said even the details released about the report already released, around which organisation was responsible for the central bank's governing legislation, amounted to a power play.

Treasury's refusal to release the report for so long suggested it did not give the Reserve Bank structure the clean bill of health the government wanted, at a time when political rivals were calling for the bank to be reformed.

"If the report was concluded in the direction Steven Joyce wanted it, we would have seen it released by now, because it would have supported him," Reddell said.

While countries such as Australia split monetary policy from banking regulation, but Rennie said it went against the international trend, which saw the Bank of England take over regulation of the banking system from the Financial Services Authority.

But Reddell said even this model would require a change for the Reserve Bank, with separate internal boards for interest rate setting functions and banking supervision.

Almost uniquely, the Reserve Bank of New Zealand governor is solely responsible for setting the benchmark interest rate, although former governor Graeme Wheeler established a committee decision-making model. All members of the committee were Reserve Bank staff, who reported to Wheeler.

Joyce has not responded to a request for comment on the report.