The council unveiled a schedule of works for the first time in a briefing to councillors on Tuesday night. Staff said the expansion could begin early next year and be completed by mid-2024.

Early works will include relocating telecommunications services from the expansion site and grouting two underground mine voids.

The council is expected to call for tenders for that work before the end of the year and will launch an expression of interest campaign for the development work in the first half of 2022.

Construction would then begin in August and run until March, 2024. There would then likely be a three-month commissioning period before the gallery reopened.

Greens councillor John Mackenzie raised concerns about the length of time the collection would not be accessible and asked staff it could be displayed elsewhere during the closure period.

"The artworks are going to be in storage for a long period of time, I wonder if there are any opportunities to make use of other council buildings or other places in the city where we can have at least some of the exhibition on display?" he said.

Newcastle Art Gallery director Lauretta Morton said "from a risk-management perspective" the entire collection at the gallery would be placed into storage.

But she said it would be available to view online and there would be activations promoting the expanded gallery during the closure.

The council said in a statement on Wednesday that the gallery could not remain operational during the expansion because "significant work" will be completed on the existing building's "roof, eastern and southern sides".

"The nature of the work required does not make it possible for the gallery to remain open during construction, or for the collection to remain on-site," it said.

"The works of art require 24-hour, international standard environmental climate control and security, which is not available at any other City of Newcastle facility.

"In order to maintain these conditions, the collection will be removed from the gallery and transferred to an appropriate storage facility for the duration of the expansion project."

The council said the relocation would take several months and be completed during the early works.

The almost $40 million expansion will double the gallery's total floor area. It includes more exhibition space, a new cafe, retail shop, educational area and back-of-house facilities.

A revised plan for the project, first approved in 2012, was lodged earlier this year. The development has the same bulk and scale with mainly internal alterations made.

The council increased its funding for the project to $16.2 million in February after the NSW Arts Minister repeatedly called for it to bump up its share.

It then passed a motion in July to seek a $22.6 million loan from the NSW Treasury Corporation.

The loan, in conjunction with bequests and fundraising, would allow the project to be completed without state or federal funds.

But a decision on the loan will not be made until an outstanding $10 million federal grant application is determined. The council has also sought $5 million from NSW government.

Despite the gallery not being in the federal electorate he is running for, Nationals candidate for Hunter, James Thomson, recently called for the Morrison government to provide the $10 million.

The gallery's collection is worth about $115 million. Ms Morton said the expansion would allow "all of our icons" to be displayed.

"The aim is to have everything out of the cupboard, so to speak," she said.

"We're loaning artworks to the National Gallery ... we're loaning them to every state gallery across the country."

Lord mayor Nuatali Nelmes said it would be a "wonderful outcome" to secure either state or federal funds.