Trust chairman Nicola Williams said yesterday that government support had been critical and getting confirmation of the funding prior to Christmas was the best possible news.
"It's also an endorsement of the national significance of the 98-year-old heritage gallery and its collection which spans 400 years of international and New Zealand art history."
Ms Williams said the project could not have made it to this point without the support of the district council and a huge financial contribution from the private sector.
"We have been overwhelmed at how people have embraced the project, against all the odds, showing extraordinary generosity that has gone way beyond expectation."
She said fundraising was continuing and the trust was confident of raising the balance of the capital required during the construction phase.
Mayor Hamish McDouall said the fundraising effort had been exceptional.
"What has been achieved so far is a staggering accomplishment and will be hugely beneficial for the Whanganui region."
Detailed design work for the redevelopment is due to be completed next year with construction timetabled to start in 2019.
