The extension and structural improvements are estimated to cost $22 million and council has applied to the Ministry of Culture and Heritage for up to $10 million in government funding for the project.
"We need to make inquiries as to why the numbers are falling, what programmes we're putting in place to arrest that so the council can show the Government the level of community support there is for the gallery," Mr Laws said.
Mayor Annette Main said the figures had been skewed following the decision by UCOL to bar any of its students from visiting the gallery until earthquake-proofing issues were addressed.
"Some other schools are still visiting while others aren't. But I agree, we need to have more specific information," Ms Main said.
A report to the council's community and environment committee said the main reasons for the decline had been the absence of the Whanganui Arts Review, a drop in the number of events at the gallery in the first few months of the financial year, and a reduction in school visits.
Councillor Rangi Wills said the impact of the engineering assessment of the building would influence the numbers but he did not see that as being a major issue.
But Mr Laws said it was worrying that the number of national and international visitors had fallen. He said it was happening at a time when visitor numbers of the Whanganui Regional Museum had been increasing. Both local and national visitors to the museum had shown marked increases.
The council has asked for some better data on visitor numbers to the Sarjeant.