OK, this will be a bit long winded, but google maps dose not cover the area well and as it is fairly heavy bush it is hard to even make out the tracks through the trees on satellite view.
From Avoca head out past the footy oval on Vinoca Rd, follow that and keep left past the Blue Pyrenees Estate winery. Follow it for a few Kms till you come to a little bridge with a large camp ground at the bottom of the hills. From here there is another track that heads arround the bottom of the hill, if you take this track after about 1Km you will come to another large open camp ground on the left, to the far side of this site there is a track on your left that will head up the side of the mountain, this is Frasers Track. If you don't take this one and keep going another hundred or so meters there is another track off to the left it is fairly tight turn to get onto it, but this is the very start of Old Bluff. I would not recomend taking on Old Bluff by your self the 1st time you go up there. It starts as just a bush track then splits into 3 from left to right insane-ridiculously hard-Easy, They all meet up about 1/3 of the way up the hill, then there is only 1 track for a few hundred meters that depending on the weather in the last few months can be anything from slightly challenging to needing a winch, it gos back to 1 track for a little bit then splits again, left is easy-med right is hard-extreme (last time I was there there was 2 steps over a foot high). Once you get past all that there is just the one track the rest of the way to the top. Once you get there if you turn left and follow the track for a bit (I think it is called Main Break Track) you will come to the top of Frasers Track. In the dry you could do Frasers up or down on 31" All Terrains, in the wet you would want 31" Muddies and possibly a winch. As for Old Bluff, In the dry I would say 31" muddies as there is a fair bit of loose stuff, In the wet it would be a whole different story. If you go up there without someone who has done it before just make sure you have a good spotter and I would suggest a winch wouldn't go astray.