Under the Auckland Regional Amenities Funding Act 2008, the 10 amenities "may not receive funding from the Auckland Council for expenses that have been or are being funded under this act".
Mr Brown and the council refused to discuss the legal obstacle yesterday, saying in a statement there will be a report in due course when Mr Brown would have more to say.
"The legislation still allows room for the trust to receive funding from council - for example through capital funding - and this will be addressed in the report. But as the mayor has said, the trust will also need to make its funding case to the council," the statement said.
Amenities board chairman Vern Walsh said the clause in the act would make it very difficult for the council to provide operational funding to the helicopter trust.
"I don't know what mechanisms he [Mr Brown] has got in place to sort that out. It is really for them [the council] to sort through," he said.
Under the act, the board receives a budget each year from the council to allocate to the 10 amenities for operational costs. Funding is not available for capital expenditure.
The $900,000 Mr Brown is seeking from council coffers is for operational costs requested by the helicopter trust, and turned down by the board. The helicopter trust, because it is successful at fundraising and has a healthy balance sheet, is the only amenity to have a funding cut.
The board has slashed its grant by $750,000 over the past two years. The trust sought $1.35 million to maintain levels of service this year, but the board believed $450,000 was appropriate.
Last night, helicopter trust chairman Murray Bolton said he had had a "very positive" meeting with Mr Brown yesterday where the mayor repeated his pledge to address the funding gap.
He said the act allowed the trust to receive funding from both the board and the council as long as it was not for the same expense.
On Thursday, Mr Brown and councillors will decide whether to approve or reject the funding board's levy for 2014-2015. If they reject the levy, the matter will be referred to an independent arbitrator who can determine only the total levy, not the allocations to the 10 amenities.
What the act says
Amenities "may not receive funding from the Auckland Council for expenses that have been or are being funded under this act''.
nzherald.co.nz
Follow the debate over the funding here: tinyurl.com/helicopterfunding