Can someone please offer up some ideas for both single and multi-day packraft options in Victoria?
Depends what you are after; packraft specific trips (ie walk in, paddle out) or just general paddling (using car shuffles). For the latter have a look at the Paddle Australia website, there are plenty of options for both single and multi-day trips. Packraft specific trips are only limited by your imagination 
It also depends on your area of interest (flat water, salt water, white water etc).