Society chief executive Wendy Houliston said 11 units would be available for patients and their carers.
The work included extending lounge and dining facilities and improving the meeting area.
It is the first time there has been any major development of the building since it was built in 1989 and the current project is the culmination of about three years' work, including fund-raising.
All the units have been closed during the building work, which is being carried out by Amalgamated Builders - this was unavoidable, she said.
The society's office was staying where it is during the project, as it was considered too difficult to shift to another site and then back again.
Usually, the society was able to offer accommodation within the accommodation subsidy of $75 a night patients are entitled to, but Ms Houliston said that with the units being out of action during construction, some patients would be out of pocket.
One of the advantages of staying at the units was that people from out of town - more than half of those who used the accommodation were from Southland - could go home for weekends without having to pack everything up.
Most patients were in the units for six or seven weeks.
Although people sometimes had reservations about being around other people with cancer, they found there were benefits when they did visit.
It also provided an opportunity for carers to talk to other people in a similar position.
Society staff could also provide more information to carers.
