Local Government Act 1993 No 30
Current version for 14 May 2020 to date (accessed 4 July 2020 at 16:14)
224   How many councillors does a council have?
(1)  A council must have at least 5 and not more than 15 councillors (one of whom is the mayor).
(2)  Not less than 12 months before the next ordinary election, the council must determine the number, in accordance with subsection (1), of its councillors for the following term of office.
(3)  If the council proposes to change the number of councillors, it must, before determining the number, obtain approval for the change at a constitutional referendum.