There are references to Australia in nearly every one of the Swallows and Amazons books involving the Walkers. Mary Walker, mother of the "Swallows", was brought up on Sydney harbour and a sheep station in Australia. Most of the references relate to her real (or imagined) experiences there (SA (Ch2, 16, 18, 31), SD (2,8,15, 36), PP(16), WD (2) and SW (particularly Ch28, an interesting interpretation of a corroboree )).
Mrs Walker's well-traveled naval commander husband, Ted, also makes the following comment when the Swallows see the four-masted barque Pommern: In ballast .... She'll have left her grain at Ipswich. She'll have come round the Horn from Australia .... (WD24).
Captain Flint's trunk has a label marked with a Melbourne hotel (SA28) on it. Peter Duck talks about Pelorus Jack as piloting vessels in Sydney Harbour (PD17); however, the real Pelorus Jack shephered vessels in Cook Strait in New Zealand.
And in GN?, Roger mentions cockatoos (Ch 12) and there is a discussion on the way the natives dealt with Captain Cook (who discovered the East coast of Australia) in Ch 21.