Auckland mayoralty contender John Banks predicts cheaper water as a result of the super-city reforms.
At a mayoral debate on Tuesday night, Mr Banks said there will be substantial savings and lower prices in merging the water operations of the seven councils.
However, Len Brown, his main rival, says cheaper water is an easy promise to make, but difficult to deliver.
All of the present councils use different charging methods.
Two other candidates, comedian Ewen Gilmour and property manager Colin Craig, both pledged regular referendums on big issues.
The new Auckland Council comes into being on 1 November. It will comprise eight existing councils.