Now that Qantas have installed or are installing check in kiosks inside the Qantas Club lounge in Sydney can they really check that you are checking in for one of their flights. Have a fake itineriary made up and then have an old boarding pass.
Only problem it is a bit of a long walk, about 10minutes, from Qantas Club in the Qantas terminal to Gate 39 at T2.
It would depend on how much time you got and how desperate you are for a coffee.
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You're only given a little spark of madness. You mustn't lose it.