I havent flown in oz for many year either but i believe the rules are similar under all icao states.
The ceiling and viz for filing as alternate are only a preflight planning requirement.. Ie you need to have the altn fuel on departure but once airborne you can do whatever you like with it as long as you plan to land with final reserve.
And remember the inflight altn minima requirement becomes the landing minima.
You can quite legally nominate another alternate in flight as long as it meets ldg minima and you can land there with final reserve fuel.
Therefore you are effectively nominating syd as your new 'altn' as long as it is at or above the ldg minima.
Our company has very specific rules regarding continuing to the destination and i would imagine every operator would have their specific rules.
By the way under the new ICAO rule we are now required to advise ATC when we are effectively comitted to land by telling them "minimum fuel". When it becomes apparent that we will land with less than final fuel reserve we must declare "mayday x3 , Fuel".