There is no way on Earth I would be on that flight to Sydney tomorrow. The absolute best case scenario is that you get shut up in a hotel room for four days.

First thing I learned as a gambler: never make a bet with no upside.

EDIT: Caveat: the only situation where taking the flight to Sydney might be beneficial is if Trans-Tasman flights get cancelled and BA keeps flying - and it defies logic that NZ-AU flights would be off, but AU-SG-UK flights on.