The last games in Australia were on the Gold Coast. I doubt that having the Comm Games there did anything to boost and area where tourism is already the major industry and which is one of the world's major tourist destinations anyway.
As for Victoria (note it is a state and not a city that will host in 2026) the games will largely be in rural cities. I can see an increase in Aussies wanting to visit but not really overseas. These rural areas have their own charms but I can't really see foreign tourists wanting to spend the limited time they usually have to visit Australia taking the time to get from Melbourne to one of these places and back again.
Most Olympic cities end up with a series of white elephant venues that aren't used much after the games, if at all. Sydney and London got it right by either using existing stadia or having temporary ones. Even the Olympic stadium in Sydney was always planned to be reconfigured from an athletic venue to a football venue, which it now is.
