Hi fillup; terrible question that I ask, but live or dead? I just bought 5 dead ones today, 16kg all up, and paid ~$9.50/kg for them (from a local fish shop). We needed them for dissection purposes, and I wasn't about to go out and collect them for this purpose.
What species are you looking for? Pinnoctopus cordiformis (= Octopus maorum of many), Octopus gibbsi (= O. tetricus of some, but I have some doubts until the systematic status of tetricus is resolved), or O. huttoni (= Robsonella australis of old, and Octopus warringa of others more recently)?
If you SCUBA and want to keep the animals alive then P. cordiformis is easy to find subtidally on soft-sediment flats, wherever you get scallops (not represented in collections from north of Whangarei); O. gibbsi is a shallow-water reef-dwelling species, reasonably common in the Auckland area, more common north of Whangarei, in clean-swept habitat; O. huttoni is not at all common around Auckland (it is far-more common in southern waters, and strangely, off the very top of New Zealand), though I believe it once was common in Hauraki Gulf (primarily subtidal on shell-armoured clean-swept sea bed). Neither P. cordiformis nor O. gibbsi would be appropriate for aquaria - they grow far too large!
What do you intend to do with these beasts?