DeanFish,
I haven't been there for about 2 years so I hope it hasn't changed. As you drive north along West Portland Road, go past River Road, over the Bridge and the turn off to the boat ramp is about 100 or 150 metres past the bridge. It is just a dirt track and if your not looking you can easily miss it. The ramp is fairly steep but I've never had a problem launching a 4 metre Quintrex towed with a Nissan X trail. Hope this helps.