Prosper Northland Trust (PNT) chairman Barry Trass confirmed that should it fall short of $16.25 million by June 30, the whole project will be "dead in the water".
"A lot of people think it's a done deal, but it's not."
The trust was charged with securing its required $16.25 million funding within two years, in the terms of the Whangarei District Council's historic 2015 referendum.
Since then, increased construction costs and greater earthquake proofing have driven up the cost by $4.7 million, to $20.9m.
Nearly $15 million has been raised to date from individuals and organisations, government departments, community funding agencies and the Lotteries Board.
Mr Trass said there was currently an application in for a Significant Project Lotteries grant.
But just as appreciated as any large grant, and even more heartfelt, was the $203 Matarau School donated to the cause last week, he said.
"Every cent counts, and what a great community example from this little school."
Meanwhile, the arts crowdfunding platform Boosted has weighed in behind the project, with Whangarei law firm Henderson Reeves pledging to match it dollar for dollar up to $20,000.
Boosted General manager Simone Hunter said the ambitious campaign had been effective so far, with at least $10,000 in the bag. (boosted.org.nz or directly at http://www.boosted.org.nz/projects/hac)
Crucial to the HAC is the Wairau Maori Art Gallery (WMAG), with its own board to look after the Maori-specific components of the project.
As the world's first gallery dedicated solely to contemporary work by internationally recognised Maori artists, it could be as big an attraction as the Hundertwasser building.
Museum of New Zealand Te Papa Tongarewa and Auckland Art Gallery have agreed to loan contemporary works to the gallery for four, three-month long exhibitions a year.
WMAG chairwoman Elizabeth Ellis said Prime Minister Bill English's recent visit to the site gave him valuable insights into the project, funding requirements and the importance of including a dedicated Maori art museum.
Ms Ellis said the project will "change the North and the cultural profile of Aotearoa NZ".
So far, no money has come directly from Maori funding sources.
Grants include $50,000 from Pub Charity, $4 million from the Government, $1.8 million from Foundation North and $1.5 million from Northland Regional Council (through Northland Inc).