You need to also consider what events down under would drive up pricing on the Aus-Cda legs- school holidays, start of vacation time, ski season. Recent example: lots of room Canada to Australia during the first week of the Commonwealth Games; very little coming back. Priced out according to the return.

Google flights will give some limited sense of pricing history for a defined itinerary and set of dates; or flag you with a price in a zone you set- not complete transparency but can give some insight into day of week low fares.