Dunedin, whose population of 125,000 is smaller than the 128,000 residents of the Howick Local Board, has added its name to the alliance along with the Otago Regional Council, and Central Otago and Clutha district councils.
Councillors from Dunedin were advised on Monday that one of the aims for the "coalition of the willing" was to promote a united South Island voice in talks with the Government.
Other goals, according to Dunedin Mayor Dave Cull, were to recognise common interests among the 27 South Island councils and finding ways of co-operating for things like shared contracts for road maintenance.
Super City Mayor Len Brown said it was great to see Auckland's brothers and sister down south working to create a Super South Island.
"They describe themselves as the coalition of the willing. In Auckland we started off as a coalition of the unwilling, but I think most of us have been convinced of the benefits of a united Auckland.
"Up here, we're now speaking with one voice, delivering savings of $1.7 billion and fostering an Auckland spirit. I hope the mainlanders have as much success," Mr Brown said.
Auckland Chamber of Commerce chief executive Michael Barnett said the "coalition of the willing" shared a population of about one million while Auckland had about 1.4 million people with one mayor and 20 councillors.
Mr Barnett rejected concerns about the Super City hogging Government funds for infrastructure, saying for decades it had contributed more than it got back. "If Auckland is growing at a fast rate it is due to demand - a problem I am sure Invercargill would like to manage," he said.
- additional reporting Otago Daily Times