Thanks everyone for the replies.
I went ahead with Air Canada Cargo, and it was a success. Here are details for anyone interested in doing the same thing.
I had 50kg of stuff to send. Too much to send by post, not enough to justify a container, so I called up Air Canada Cargo and made a booking. Dropping it off at Edmonton airport, I had to provide contact details, the packing list, and $320 (charged by weight, not volume... nice). There were no Canadian customs export charges because the total value was less than $2000.
In Sydney, the collection process was:
- Go to the Air Canada Cargo office (near the international terminal), pay $110 in fees and collect some paperwork.
- Take the paperwork to the customs office (walking distance), fill in form B534, and see a customs officer. I should have paid customs duty and GST on everything in my shipment less than 12 months old, but since I only had a few low-value things in that category, the customs officer kindly waived those charges.
- See a quarantine officer in the same building. I did report some food and wooden items, but the officer decided an inspection was not necessary. Stamp, stamp, paperwork done.
- Walk back to the Air Canada Cargo office and wait around for them to pull my stuff out of the warehouse.
Total cost: about 440 Aussie dollars