He said the first signs of the damage came after the Kaikoura earthquakes, in 2016, when the team noticed a few new cracks in the building.
"We did a rapid assessment and that stressed further investigation was really important."
The building had been crack-mapped in 1995, but an assessment last year found about 140 new cracks, he said.
Maintaining the building's heritage has meant every aspect of the work being undertaken on its repair has been carefully considered.
"It's particularly challenging with the geothermal landscape and the fact the building is in three parts, so how the structures actually work together.
"Some of those unique features which were part of the original building techniques are now representing the biggest challenges."
The original structure was made primarily of pumice in a process which utilised the local landscape to produce concrete.
"There are also areas of barbed wire, which were noted as being of a 'reputable brand'," Brown said. "We did scans and in some places, there were cavities of air in there.
"The big difference between now and then is we know more now about seismic activity."
Extensive structural and geotechnical assessments were undertaken to understand the work required.
"We're trying to get a clear image of what we're dealing with, so that in 50 or 100 years from now they'll be saying, 'Look, they did a good job'," Brown said.
He said the maintenance issues the original builders had were pretty severe because of the hydrogen sulphide in the area.
"All of the original copper nails have corroded away, so it's also about sourcing the most resilient products we can and thinking how we can reduce the future maintenance costs by being smarter now."
The architectural concept design is currently being finalised in a collaborative approach with input from engineers, heritage architects, geological experts and museum staff, and will be presented to the council in the near future.
Construction is due to begin in 2019 with proposed completion in 2021.
"It's a real challenge to come up with a design concept that's going to work," Brown said.
"We don't want to hide it and there'll be some form of exhibition where people are able to see the steel and the work that was done to get the museum to its next stage of life."
He said he would like the building to still be a well-used asset in the community in another 110 years.
"Still playing a role for visitors experiencing what's special about Rotorua, highly utilised and highly loved."
Rotorua mayor Steve Chadwick played a part in the completion of the building for its centenary in 2008 with her government role in arts, culture and heritage.
"I'm very passionate about the building. It's a real local treasure and also a national and international treasure."
She said it was "really, really exciting" when the council went out on Long-Term Plan consultations to see how connected the community was to the museum building.
"That confirmed what to me has always been an emotional attachment to the building."
As the council worked through the restoration process, she said there were some "amazing complexities".
"It's had an assault on its undercarriage, this heritage fine lady, and it's a huge challenge of engineering, location and heritage.
"The building is what it is, a historic building of huge importance and it needs to be restored as that. We think it's going to cost about $30.5m."
She said in the future there was a very exciting story to be told about the restoration.
"In itself, it's a very important part of the history of the building.
"Every single day I sit in my office and I look at that building and it inspires me as I work, and it inspires me when I go home to and it's lit up by the lights, when she puts on her evening clothes.
"I see the thousands of locals and tourists that are out there walking the grounds or taking photographs and I can't wait for the day the doors can open again."
