Refueling airplane, can you get off and go to duty free?

From Heathrow to Singapore that is considered one flight. Then from Singapore to Melbourne is another flight. You will be required to get off at Singapore and then wait in the waiting area until it is refueled and they commence to board the plane again with some new passengers. Also, they will most likely be more expensive at the airport so just buy them before you go into the airport at Heathrow! For the most definite answer just call Qantas and ask them about it! I'm sure that they'd be more than glad to provide an answer!

If it were just a refueling stop, you almost certainly would not be. But it's not. Your flight, which is QF10 by the way, is scheduled to be at SIN for 1.5 hours. Passengers will be getting off, and new passengers getting on. I expect you can leave the aircraft. You might even be required to. But for a definitive answer, as Qantas.

There is a good chance because I have been on a few flights (operated by different airlines) where they want everyone off the plane so they can clean it as well. What everyone had to do was get their carry-on luggage off the plane as well.

On long flights,you will be allowed out of the planes

Both airports are super expensive.