Thought I did post an update:

I popped into my local fishing store today and ended up buying a TLD 25 on a 15kg Shimano Backbone Elite, spooled with 300m 50lb braid.

I considered purchasing the 24kg rod briefly but it was heavier and stiffer (heh) for what I would have used it for, which would mostly be ledger or running rigs with bait.

If anyone is in West Auckland, I highly recommend popping over to Norcross Fishing World. Very helpful staff and always nice to support local businesses.

Once again, thank you all for the advice!

It is hard to know. I'm not saying it's easy. Motor up current, drop your gear down fast with the aforementioned 8 or 10 oz sinker. Let out more line than you think would be required. When you start getting bites, you know you are on the bottom or close to it! Graveyard snapper aren't shy, you'll recognise the bites. 

Another tip. Within reason, I don't think you can fish too heavy in that area. 100lb leaders and flasher-style rigs with 8/0 hooks wouldn't be unreasonable. Double hook ups of good size snapper are common, plus the sharks are really hard on the gear.

I'm pretty sure the pressure on the line will lift the weight off the bottom. Those breakout sinkers are more designed to stop rolling I would think.

 

I tend to use light gear & braid in the harbour. High current areas need sturdier stuff, such as the TLD's and a rod to match - braid will help a lot too. 20 to 30 oz's of lead isn't an option on lighter gear.

Alan