A Whanganui Community Foundation donation of $1 million will help get the Sarjeant Gallery redevelopment project over the line, says gallery trust chairwoman Nicola Williams.

Provided written pledges to the project can be finalised by the end of this month, she is confident the trust will raise enough to unlock a Government contribution of $10 million toward the $34 million project.

And getting to that total should unlock more funding still, she said.

The Community Foundation approved a grant of $250,000 to the Sarjeant project in October last year. The trust then asked for a further $750,000 to ensure the funding deadline was met.

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Foundation chairwoman Alaina Teki-Clark said members agreed from the get-go that the foundation would give something.

"It was just how we would work it in with our finances."

The foundation's $1 million donation will be paid over three financial years from 2016-2019, and Ms Teki-Clark said the Sarjeant's redevelopment project would strengthen and restore the existing building in Queen's Park and add a new purpose-built wing.

The Sarjeant Gallery Trust already has more than $22 million in cash and written pledges, Ms Williams said, but the written pledges would need to be finalised to match the $10 million in Government funding.

The $22 million includes $5 million from Whanganui District Council, while the $10 million needed to match the government contribution must come from independent sources and the council's money cannot be part of it.

The end of June funding deadline is crucial, Ms Williams said, because it has already been extended twice and Arts, Culture and Heritage Minister Maggie Barry might not extend it again.

Ms Williams is determined to meet the deadline, and said the building project could begin as early as mid-2017.