The walk from Mt Erica/Mushroom rocks to Mt St Gwinear/Phillack is relatively easy. The trail is well worn, and marked, so you should not have difficulty following it. The trail will cross a stream at the T intersection (of the off the AAWT) that leads to the Talbot/Eric Hut ruin (which is just a chimney). You can’t miss it, as there’s a footbridge over it. As already mentioned, this is a half-day walk ...and unless you spend a lot of time exploring mushroom rocks (btw water in a tank at the scout hut), you may want to consider pushing on and camp at the ruin a few km south of mount Whitelaw. About a 8 hour from the carpark and on fairly flat terrain. The campsite here is similar to Mt Erica with a chimney, and has water. From the ruin, there's a well worn trail that leads down (east), behind some rocks and into the bog ...and leads to a running stream. About a 75m walk from the chimney, if that. The water is fresh (potable) and fairly reliable even in summer.
For navigation I use the gaia ap on my smartphone (was iphone but now using android). To conserve battery make sure it’s in airplane mode and turn on only when needed. I get about 4 days per battery charge (and carry a spare battery and compass + map). Pre download the maps you need, so you will not need mobile coverage/internet. Test everything before you go. There is coverage (about one bar) through most of this area, but drops off around Mt Whitlaw hut. However, I would not rely on this. It's fairly remote, often with dayhikers round the St Gwinear/Baw Baw section, but I would not plan on seeing anybody.
If you are using a GPS for navagation, or looking for info on the walk I recommend visiting Karen Cody excellent AAWT trip summary at
http://www.johnevans.id.au/Pages/KC/KCAAWT.html. Download the KML files for day 2 and 3 into Gaia, and you will get waypoints and info on the walk (including terrain profile). Then use Gaia to download the (track) maps for offline viewing. The site also has some references to paper maps for the area you are walking.
As mentioned, the drive to the carpark is easy. A dirt road, uphill, a few potholes and dropped branches, but nothing a (read my) clapped out Ford Metro can’t handle.
The walk from the Mt Erica carpark, north to Mt Whitelaw is perfect for a first overnight trip. My personal opinion is that it's possibly one of the the best in Vic.
FWIW I spent Easter walking from Baw Baw Village, north to Thompson River (and back), and with the cooler weather drank less than 2 litres a day. It was fairly cold overnight, so plan for +5 plus wearing all insulating layers, or a 0c degree bag. It may (actually it will) be fairly damp, so bring rain gear. Interesting that when the cloud rolls in (mist/fog whatever) the trees start raining. The clearings will be dry as a bone, while the rain is literally 'down pouring’ from of the trees. That's what makes the high country magic.