Wellington City Council's general manager of building control Mike Scott said turning off all the services was going to be the biggest issue over the next few days.
"There's a generator in the bottom of the building in a substation," Scott told gathered media on Monday.
They needed to figure out how to switch off the generator without sending anyone inside, he said.
The demolition contractors were bringing in a 110 tonne machine from Christchurch to help with the "deconstruction" of the building.
"They actually deconstruct the building from the top and work their way down, it's a very controlled and measured way of undertaking the deconstruction," Scott said.
He said the demolition would be a "relatively quiet process".
"It's actually a set of jaws that crunches the building down, you won't hear huge jackhammer noises or anything like that," he said.
Scott confirmed there had been tenants living in the building before the quake, despite the building not being registered as residential.
Council had been unaware of the tenancy before the earthquake and were now working with affected tenants to support them, but he said any decision on action being taken on the person responsible for renting the space to them would come later.
"We're actually working on the immediacy of the issue at the moment," he said.
"We've done over 100 hours over the last few days working on this, my team are fully stretched making sure we deal with the immediacy, public safety, and making sure this building gets sorted out. We will deal with that, we have plenty of time to assess that."
Scott said Wellington had fared well in the earthquake, which hit particularly hard despite not being on the Wellington fault line.
"You can't design for every single eventuality," he said.
"The analogy that has been sort of explained to me if you look at a train track the actual epicentre of the earthquake was a train leaving the station. It shot past Kaikoura and caused damage there, and the end of the train line was actually Wellington city. So as a result we did take a fair whack.
"It was a significant earthquake but as I say, we've actually come off pretty well."
The Statistics NZ building, which was another of the worst hit buildings in Wellington, remains under investigation.