I don't know about the TAF at departure time but I was flying that night (not to SYD) and heard some A/C (BA and few QF) reducing cruise speed due to the forecast tailwind on 34L.
Pulled up an ATIS and TTF on the ACARS and there was a wind of around 25 kts from the south.
I would have thought the fuel required for 30 minutes was closer to 4 tonnes but either way he must have been very "thin".
