Australian here! Count that as half good as I understand how customs works here, but half bad because I've only ever entered on an Australian passport rather than a foreign one! Thus my experiences being screened are quite possibly not representative.
Yeah, this is a bit of a hard case. It's technically a requirement to declare it if the value is >$900 AUD (690USD/890CAD) and you have owned/used it for less than 12 months. No Duty/GST would actually be payable, as you only plan to import it temporarily[1]. In reality this is rarely followed to the exact letter of the law, as many people liberally interpret the 12 months part or think goods only refer to something new and re-saleable, or with intent to sell, etc.
Though relatively uneventful, obviously declaring it would be a slight pain on account of not revealing the plan to the future Mrs Eternity000! Sadly they also tend to keep couples together when doing passport control, screening, etc.. making it difficult to wing it on the side.
I believe general intent is to stop people avoiding Duty & GST by way of personal airplane courier. We have a similar limit (which for some reason, is $1000 instead of $900) for importing goods via mail from overseas - i.e. every online purchase and there are similar rules for importing your personal goods (even as a tourist) that do not travel with you (unaccompanied).
The reality is that, unless you have other goods or large quantities of goods that are going to draw attention - a ring in your luggage (or carry on) is incredibly unlikely to attract attention, even during a search. However there is always a chance it would be exposed during during a screening search of either should that occur for some reason because you are likely to be together. Having the ring is unlikely to trigger that search, but it may well be simply moved around / unpacked / exposed during such a search - not just at the AU border, but at departure, or at security checkpoints if in carry-on.
The way the screening process works in my experience is that when you turn in this card, you have gone through passport control, collected your luggage (and have it with you) and then an officer checks your card and will either wave you through with no further checks, or may put you into a line to have your baggage x-rayed and potentially physically checked -- or sometimes if you declared something they'll just get you to unpack the declared items to check and or discuss them. A good amount of the time you just get waived through as an Australian, and from others in the thread seems likely as Canadian/USA - the most common problem they are dealing with is people bringing in food (we have strict quarantine laws compared to many parts of the world, especially europe/asia).
For the proper information page, see:
https://www.border.gov.au/Trav/Ente/...ee-concessions
Personal goods are free from duty and tax if they are:
- owned and used by you overseas for 12 months or more
- imported temporarily (a security may be required by the Department)
If you don't want to live free and dangerously (which honestly, would probably just work out) and ensure you are 100% legit while also not revealing the master plan, the best suggestion I have is to import it via post to the hotel or something like that. Then you can do all the declarations ahead of time. Not sure I could stomach having it out of my site though! See:
https://www.border.gov.au/Busi/Impo/Send