Initial estimates to strengthen and extend the Sarjeant Gallery have jumped by more than $4 million, with the figure now sitting at $32.45 million.
Two years ago the work was set at $28 million.
Nicola Williams, chairwoman of the Sarjeant Gallery Trust, responsible for fundraising for the project, said inflation, a refined budget for professional fees and an increased contingency allowance had all contributed to the increase.
Ms Williams said the Wanganui District Council had taken a "conservative and cautious approach" in assessing the costs and for that reason had made greater allowance for contingencies and other provisions.
But she said every effort was being made to find savings "and we aim to keep the budget as close to $28 million as possible".
She said more detailed designs had "clarified" the costs of things such as earthquake strengthening but gave an assurance that a "robust contingency" was in place to ensure the project did not go over budget.
"There are some costs involved in securing funding and, consistent with other projects of this size, the fundraising campaign is estimated to be between 1-2 per cent of the total project cost."
About $14.3 million has been sourced, including funds in the bank or pledged in writing and $10 million central Government has set aside for the project. This leaves a further $18 million to raise.
Ms Williams said, given the Sarjeant's national significance, the trust "strongly believed" this was achievable.
She said fundraising would take care of any increase in the project's budget.
The fundraising effort was gaining traction and the response from the community at recent public workshops had been more than encouraging, she said.
"Within the last two months alone and, based wholly on the public presentations given by our working group, there have been two major pledges from trusts totalling $2 million. With the project falling into place, discussions with other potential funders are looking extremely promising."
She said with that groundswell of community support, the fundraising campaign was "steaming ahead" and the trust is "rolling out a revenue generation strategy to get the remaining funds in the door".
Ms Williams said there had been a very positive reception to the redevelopment project.
"Wanganui people understand the significance of the Sarjeant Gallery Te Whare o Rehua Whanganui and its collection [and] people are beginning to see the significant economic benefit that the redevelopment will bring to Wanganui, both in the short term and in years to come."
She said by completing the developed design and applying for resource consent, the district council had provided certainty to the gallery project, which she called "a once-in-a-generation opportunity".
She said the project timeline remained the same, with an opening set for September 2019, in time to celebrate 100 years since the Sarjeant opened.
Mayor Annette Main said it would be "an act of civic vandalism" if the city turned its back on the Sarjeant and would have Wanganui "back in the news for all the wrong reasons".
While the country struggled to find affordable ways to bring heritage buildings up to acceptable standards, the Sarjeant project gave Wanganui the chance to become known as a centre of earthquake design and engineering excellence, she said.
"We already have some talented local people who want to be part of this, and the work that turned the Sarjeant on the Quay into such a low-risk and functional building shows we have the expertise available."
But Ms Main said although the temporary gallery was a good investment, it would never replace Queen's Park gallery, now almost a century old, "as a beautiful and aspirational place that stands apart among provincial New Zealand towns and cities".
At last week's operations and reporting committee meeting, councillor Philippa Baker-Hogan expressed concern at the overruns.
Mrs Baker-Hogan said the costs have now been put at $32.45 million when "it's an activity with less than 50,000 visitors a year".
"The relocation to Taupo Quay is obviously a popular one and people are saying why do we need to go back to Queen's Park? We need to be able to say that is what the final figure [for the project] will be," Mrs Baker-Hogan said.