Hey mate, not sure if you’ll read this before you head up, but anyway:
My wife and I literally returned today from our first 4WD trip to the High Country. It was part amazing and part frightening to be honest.
So first of all, we mostly followed treks from this book, which is pretty good. And I think you can actually get it at the Mansfield visitor centre. The track ratings in there are mostly identical with the Hema ratings. An exception is the Zeka Spur Track, which leads down into the Wonnangata valley. Hema rates it difficult, but we had no issues going down after two days of rain, and also no issue going back up the next day (and I don’t have too much 4WD experience).
Why did we go up again the next day you ask? Well, the track notes in that book describe the aptly named Dry River, which you need to cross when entering the valley, as „usually dry“. However, it had probably 40-50 cm of water. That in itself was not a problem, but the book also says that the river crossing at Eaglevale can be challenging after some rain. So we decided to not take the risk and turn back. What also played into this decision was the fact that the night in the valley was quite scary. We had seriously underestimated just how remote and lonely the place is. So we decided that we‘d rather go back to a campground where we knew that there would be other people.
Speaking of remoteness: keep in mind that there is absolutely no phone reception down there, and UHF range is also not great due to the mountains. So if something happens, it might be a while before someone comes past. We carry a personal locator beacon just in case.
I use an app called New Tracs. I don't always trust the difficulty ratings it gives the tracks, so use caution.
https://play.google.com/store/apps/details?id=au.com.newtracs.Newtracs