Jeff,
Go to citylink and pre-buy a pass (buy pass) for the weekend and one for the Monday. I assume that you would only need one for the Westgate freeway tunnel and Monash Freeway. More info is available at the following website.
http://www.citylink.com.au/If you were to use the Eastern Freeway (Ringwood to Frankston) then that would be a different tollway (Eastlink).
See you at Hazelwood.
Regards,
Peter