Reliable information? Probably a call to the Australian Tax Office is the only way you'll get that. However, as I understand it:
1. Australian registration usually tells you that the yacht was either built or bought new in Australia. In this case it appears that the owner was Australian but bought the yacht outside the country.
2. If the yacht has never been to Australia then GST is applicable on import, unless it has been paid previously. If you never bring the yacht to Australia then GST won't apply.
3. The yacht could still visit Australia without paying GST as long as the intention was to leave again. I'm not sure how long you could get a "temporary import" for but it's probably the same as for visiting yachts from anywhere else.
"The cure for anything is salt water... sweat, tears, or the sea" -- Isak Dinesen
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