"But we expect those relationships we've developed and maintained with potential funders around the country to start bearing fruit very soon."
He said they were a mix of corporate and individual funders. He and trust chairwoman Nicola Williams had been on the road following up on those connections "and without going into the detail at this stage we're confident of some very good results before mid-year".
The Whanganui District Council had committed $5 million to earthquake-strengthening the gallery.
That work and the new extension are estimated to cost $34 million in total.
Mr Anderson said working with potential funders takes time "and people who give do so when they're good and ready, just like the last $1 million donation we received". He said that was the result of a few years work, "then it just happened".
"The same thing has happened with others and will continue to happen come the July deadline. They know what our deadline is and we're expecting those commitments to be in place by then."
He said until those commitments had come through and they got permission from those donors to make the announcement "all we can do is talk about this in general terms".
"A lot of those we've been talking to want to maintain some confidentiality so we have to respect that."
Recently, the campaign was boosted by a $100,000 donation from a Wellington woman with family links to Whanganui. Mr Anderson said while the woman had lived in the capital for more than 50 years, she had fond childhood memories of visits to the Sarjeant.
Another $30,000 was donated by Judith Anderson, a great-great niece of gallery founder Henry Sarjeant. She made the donation after a recent tour of the Queens Park building.
There had been other donations of more than $20,000 from Whanganui residents and from others around New Zealand.