In New Zealand, all local body elections are held by postal vote. That means you need to fill in your voting form and post it back so it gets there before 11th October. Or you can drop it into a ballot box at a CCC library, service centre, pool ... or a New World or Woolworths (Countdown) supermarket.
More information is at https://www.votelocal.co.nz/information-for-voters/
If you are registered to vote in Parliamentary elections (a Parliamentary elector), you are automatically enrolled as a residential elector to vote in local authority elections. The address where you are registered as a Parliamentary elector will be in the council district for the local authority elections.
You will receive a voting document in the mail so that you can vote by post.