I went to the US in October, claiming TRS on a camera ($1200), wedding ring ($2500) and set of Bose QC25 headphones ($330). On my return, ticked the box ">$900". I presented my card to the Border Force (BF) person on exit, he looked at it and asked me "what have you got?". I replied, "camera, wedding ring, QC25s." BF person "Great headphones, aren't they?" and waved me through. In Jan 2015 my son claimed TRS on a Macbook Pro, declared it coming in, was asked if he'd used it, said yes, off you go was the reply.
In the olden days before e-kiosks and smart-tickets, we took a family holiday and I had claimed TRS on a new dSLR & lens ~$3200 (and memory cards ~ $300 on a separate invoice). I ticked the ">$800" box and the lady at the immigration point asked what have you got. I told her SLR and lens because they were the expensive items, totally forgetting about the memory cards (expensive in those days!). I said I thought I was under the pooled family allowance (5 of us @ $800). I was shocked to be then told that the three kids were only "worth" $400, so my allowance was actually $2800 and that the total of my purchases was actually about $3500 (she could see my claim on her screen obviously linked to my passport number). No matter, customs officer said, you've been honest, off you go, no charge. WHEW!
With the large numbers of people entering Australia and the levels of automation now on incoming arrivals, I guess it'd be easy to not claim and you'd probably get away with it. Much like touching on with a myki - I've not been checked in the past two years, but still I ALWAYS touch on. Likewise, I always declare ">900" and have never had to pay back GST.
YMMV.