Even if their bags have been checked through, it should be possible to get out at SYD. They just need to get an immigration / customs card from the FA on board, fill it in, then explain to the immigration officer what they are doing. They should be allowed to go through so long as their visa (if not on an Oz passport) is still OK. Their only issue will being to watch the time so they can get the train into the city and then back out again.

My suggestion is they go to Circular Quay Station and stroll around the harbour & The Rocks for a few hours then head back out to the airport again.

Now this is my logic of the matter, but others may have other opinions.

BTW, its a beautiful clear, crisp Sunday morning in SYD.