Elizabeth, Hi. Welcome aboard!
You will need the "Circle Pacific" option, rather than the Circle Asia - Circle Asia is a fare that involves travel via Sth West Pacific, Far East and Asia only, whereas the Circle Pacific includes those areas plus North America. Having said that, Toronto is not a valid city for the Circle Pacific and you would need to start and end the trip in any of Vancouver, San Francisco, Los Angeles, Seattle or Honolulu.
Options ? Hmmmm. Well here's mine - am sure there will be others from other posters.
1) - Buy a cheapie to/from YVR/SFO/LAX etc to link with the Circle Pacific. The Circle Pacific will set you back CAD6469.00 or USD4725.00 in Business Class - although you may find better deals through consolidators like TourEast etc.
You would also need to buy seperate tickets to/from Siem Reap as the only Star carrier serving there is Asiana from Seoul. (A bucket load of cheap options using carriers like Tiger Airways, AirAsia, JetstarAsia etc - mainly from Singapore though). You would also need to buy cheap seperate tickets to get to/from Tasmania. The rest of the itinerary would fit though.
Pending mileage burned up, you could always go a Hong Kong transit Seoul to Siem Reap, buy a cheap one way to Singapore, rejoin your Circle Pacific trip up to Bangkok (from Singapore) and continue on as before.
2) - Get your own discounted business fare from Toronto to Hong Kong and return, then use the Circle Asia pass from there. Again, you will need to do "your own thing" to/from Siem Reap and Tasmania and return from New Zealand to Hong Kong.
Given the combined cost of the two fares (yours and then the Circle) this option is probably not preferred. Unless you use your points (I'm presuming Aeroplan?) as alluded to in your post for YYZ-HKG-YYZ.
3) - Get a RTW fare and go east from Toronto first, through Europe to Hong Kong and then follow the rest of the itinerary.
Business Class fares for the Star1 (29K miles max) from Canada are CAD9599.00 published, but a consolidator may have better.
Someone else will have an idea that I haven't thought of no doubt - stay tuned !!
Cheers/Regards.