Waiheke Island and Rodney residents will learn today whether their bid to break away from Auckland Council has succeeded.

The Local Government Commission is due to announce its decision today.

If successful, it could carve off about a quarter of the super city's landmass.

The Our Waiheke group has been campaigning to have a unitary council, arguing issues on mainland Auckland have limited relevance on Waiheke Island.

After a petition to separate from Auckland City Council, a referendum was held in 1991. The vote failed and Waiheke stayed part of Auckland City.

In 2008, the Royal Commission of Auckland Governance received a number of submissions opposing Waiheke being part of the proposed Auckland Council.

But the application has since been renewed, along with Rodney's.

Bill Townson of the Northern Action Group, which represents Rodney residents who are campaigning to break away, told Fairfax he was not optimistic about the group's chances.

"We've become cash cows for Auckland."

His group planned court action if the bid failed and would push for a referendum on the issue if its bid was knocked back.

If successful, the break away would see Auckland Council lose a quarter of its territory to a new North Rodney Unitary authority and Waiheke's bid would create the country's smallest territorial authority by population, Fairfax reported.