The group raised $500,000 and has restored its cab and exterior walls.
Its lease with the council allows it to occupy the tower and develop it into a museum and education centre, secretary John Henderson said. The provisions of the lease allow the council to take over parts of the tower if aviation requires it.
Visits to the cab at the top, with its original control console and sweeping views, were a highlight. They will still be possible when the new service is installed, but numbers will be limited and the console will be moved to a lower floor of the tower.
This disappoints some members of the group, Henderson said, but is "the best of both worlds".
"The fact that the tower can be used for the purpose it was designed for is actually a good thing. It's a bit of a win-win."
The UNICOM service will operate from the cab, in a relatively independent way, on 363 days of the year.
Architect Bruce Dickson is working out what else is needed to complete the renovation of the lower floors. When plans are final they will be costed and decisions will be made about how the cost is shared between the group and council, Henderson said.
The group will move unwanted items out of the tower next month. Anyone wanting to help can ring Henderson on 027 315 1032.