Hi Shell,
There is a definite shortage on the Net. We were able to get some maps from the NRMA (or RACQ there), but much better info is available locally at the Visitor Info Centres. There is a big well stocked one in Lorne.
You really need this map
http://www.parkweb.vic.gov.au/resources05/05_1787.htm
which they want to sell you
You can make some headway with this site
http://www.otwaywalks.com/front.html
and by using Google Maps close up. A good trick there is to select the Terrain option, which does a better job of showing you grades.
Cheers