Whanganui Regional Museum will be closed for more than a year for seismic upgrading, which is due to start in January.
In an update, the Whanganui District Council's deputy property manager Leighton Toy said the museum was the third public building to undergo earthquake strengthening, after the Opera House and Alexander Library.
But, Mr Toy said, although strengthening the building was expected to take four to six months, it would take longer than that to recreate the "museum experience" once the seismic work was finished.
Councillor Hamish McDouall expressed concern about the timing of the closure. He said it would be preferable if the museum was closed during the low tourist season rather than across two high tourist seasons.
But Mr Toy said there was a health and safety component involved as well, and that was why it was easier to close and operate from temporary headquarters in Ridgway St in the old central Post Office building.
He said that, as well as strengthening the building, the work includes reinstating the building as much as possible to its original look.
The museum consists of two buildings. The first was built in 1928 and meets only 6 per cent of the national building standards. The newer wing was built in 1975. It was originally a carpark until converted into part of the museum. It has a 30 per cent rating.
The seismic upgrade includes tying the inner and outer brick walls together, installing extra columns next to existing columns, replacing vulnerable blockwork and bracing sections of the ground floor walls to the floor.
Mr Toy said the council's 10-Year Plan estimated the upgrade would cost $750,000 but, through lessons learnt during the Opera House and Alexander Library projects, that figure was raised to $1.2 million in the 2016-17 budget. However, a more recent quantity surveyor's report has raised the estimate to $2.26 million. That includes $1.7 million for seismic work and another $530,000 for other work. The bulk of that "other" work includes $233,000 for the roof.
He said within that estimate were two significant costs - $485,000 for relocation of the museum during the work and another $261,000 for professional fees.
The relocation would cover removing and then reinstating all exhibitions to a similar standard prior to the project starting.
"If the reinstatement exceeded council's estimate then this additional cost would rest with the museum. The $485,000 estimate is council's 'line in the sand'," Mr Toy said.
Only a small portion of the artefacts will be moved during the work, with the majority packed and temporarily moved throughout the building.
Mr Toy said while the estimate was $530,000 above the budget, council has sufficient funds in its seismic upgrade budget to cover the shortfall.