Ms Williams said support for the project - particularly at regional level - reflected a growing commitment to make sure the heritage gallery and its nationally significant art collection were preserved and developed for future generations.
"We are so close now, but the June deadline is looming and we are urging individuals and organisations who want to contribute to get in now."
The S.M. Davis Trust was set up in 1957 following the death of a Whanganui philanthropist who left a sizeable sum of money to a number of local charities. The residue of the estate was invested and held in trust for the benefit of the citizens of Whanganui.
Davis Trust solicitor Richard Austin said the trust had been a regular contributor over the years to various Whanganui initiatives and Mr Davis had had a deep affection for the Sarjeant Gallery and had left a legacy to the gallery in his will.
A decision had been made to wind up the 59-year-old trust and Mr Austin said the trustees saw the redevelopment project as being of such significance - providing a material asset to Whanganui residents - that it was making a full distribution of the remaining capital.
The donation is conditional on the redevelopment project going ahead.
The proposed redevelopment includes quake-strengthening and restoration of the Sarjeant Gallery and development of a new purpose-built wing for storage of the extensive art collection, exhibitions and for education purposes.
Developed design drawings by Warren and Mahoney Architects have been prepared and resource consents granted. If everything goes to plan, it is hoped to get construction under way by the middle of next year, enabling the gallery to be reopened in time for its centenary celebrations in 2019.