What a helpful bunch you are, thank you.

The reason I thought we weren't eligible is I read Sydney Airport's own website which says "Qantas Seamless Transfer - If you are transferring between a Qantas domestic service (QF400 - QF1599) and a Qantas international service (QF001 - QF399) in either direction, you may be eligible to use the Qantas Seamless Transfer Service. For more information and to confirm your eligibility, visit www.qantas.com or phone 13 13 13."

Silly me! Should have looked further at Qantas's website....