I don't do a hell of a lot of surfcasting but i have a few ideas for you. (hopefully a few others can help you out further)
To get to the harbour mouth, you drive south out of town and take a right turn towards the Marae. This road takes you up to Wainamu beach which is near the mouth. You can park near a toilet block or carry on to the end of the road which leads to the ocean beach (Ngaruni)
I have had a few good catches of Trevally at the harbour mouth, also Kahawai and small Snapper. Last time i was there i hooked a freight train which turned out to be a stingray that jumped clear of the water after breaking the line (and nearly spooling me!)
It seems to be better closer to the mouth - on an incoming tide.
Nearer the toilet block is a deep dropoff with lots of snags, so best to use a form of sacrificial sinker there. I have found a whole pilchard with a running ball sinker does OK.
Papanui Point near Ruapuke is another possie - but i haven't been out there for yonks.
It seems to fish well, but be careful as a few have drowned off the rocks.
That might be a likely spot to send out a live bait for a Kingie?
There is a local operator with a small boat (blue covered roof) that will take you over to the opposite side of the harbour and pick you up at a pre-arranged time - for a pretty reasonable fee. (he does tours of the harbour)
We had a couple of good catches over there close to the harbour mouth.
I spend more time spearing flounder now than surfcasting, but always keen to flick out a bait and see whats around.Let us know how you get on - or send me a pm if you are keen to catch up.