"To put it in simple terms, there's a hell of a lot of people who turn up to look at the toilets in Kawakawa," Mr Key told the 200-strong crowd.
"I'm pretty sure you're going to get a hell of a lot more people coming to see what will be a magnificent structure."
The event was not without controversy.
A group dressed as barely discernible sex toys serenaded the Prime Minster from a nearby boat, with their concerns over the state of Northland rail.
They were largely ignored by people attending the sod-turning. Other protesters chose to stay away, with Whangarei District councillor Tricia Cutforth saying she would rather "stick hot pins in her eyes" than attend the ceremony.
"My only hope is that the folly will be the sole legacy of Whangarei's sorry Hundertwasser story," she said earlier this week.
The Prime Minister addressed the controversy which dogged the art centre and congratulated volunteers for sticking with it. "It takes a bit of courage to back a project that costs quite a lot and is always going to test people's sense of whether it's the right decision or the wrong decision. The reality is success has many fathers so once it is built and people are pouring into it people will get behind it. You've just got to get through the work you're doing at the moment."
HAC project team leader Andrew Garratt said despite more than $10 million raised, "there was still a long way to go".
"We need the community to get behind us," he said.
HAC team member Barry Trass said the 16-week construction of Te Kakano would start within a month and would use local tradespeople, including expertise from the build of Kawakawa's Hundertwasser toilets.
Mr Trass said the koru-shaped folly, with a 6m diameter, would become an attraction in its own right and would be a great addition to the Hatea Loop walkway.
