My best guess: Due to the forecast fog for Auckland this morning (it was PROB40), SIA297 held east coast Australian airports as alternates, hence the extended holding time due to the amount of fuel they had on board.
A number of aircraft were holding north of Nelson at high level in case they had to go to WN or back to AA. That way they didn't have to go all the way to CH and then truck all the way back up country.
CH is already fogging in again and it is forecast for AA again too.