As far as I understand it (and someone please correct me if I'm wrong)! Christchurch is a tech stop for the SCL-SYD route - in essence, this allows Qantas to leave Santiago with only the bare minimum required fuel (often less than is actually calculated as required to make it to Sydney safely), and employ tactics such as step climbs, finding tailwinds/lesser headwinds and the general lightening of the aircraft due fuel burn to use less than planned, and end up with a build up of unused fuel.
Somewhere out off the coast of the South Island on the track thru to SYD, they have their Diversion Decision Point (DDP) where they make a final decision as to whether or not what fuel they have accumulated is enough to now meet the FUEL REQ. (required) figure to the destination.
So, on Thursday, with the inclement weather in Sydney, there would have most likely been an extra requirement for holding in place (due to the backlog of traffic trying to get in). This would have taken the FUEL REQ. above the remaining quantity in the tanks - hence the lob into Christchurch to pick up some Jet A1
Why do they use this procedure in the first place? Well, if they were to put the required amount in to ensure there was enough fuel in the tanks to reach SYD, the service would be uneconomical as they would be unable to carry any real payload of Pax/Cargo with the sheer weight of fuel on board.
Hopefully this clears up why they dropped in? And if someone with more practical experience wants to correct me, please do so! Qantas 28 sure did look spectacular on approach Thursday evening in the sunset light as I was driving back South
