Thesauruses aren't really a good way of acquiring words; most of what you'll read will either be pages and pages of words you already know, or unnecessary jargon/ technical terminology that won't help much. To read a thesaurus for vocab, you'll need a dictionary too, since most of them won't have the definition of all the alternate words it provides. However, they can be really useful tools for expanding vocab if you use them right.
Also, depending how these 'tomes' are structured, you're probably not going to enjoy yourself. Alphabetical ones are kind of pointless since they repeat entries like crazy. If you can get your hands on a Roget's Thesaurus then that's probably your best bet. It's organised in a logical manner (kind of split in two, don't worry, it's easy to work out) and much more enjoyable to read.
Health warning though: as someone who used to read dictionaries, it's hella bad for your eyes. Like, I'm pretty sure that's the reason I have to wear reading glasses and get intermittent headaches nowadays because that tiny print and diacritic annotation is really tough to get through.
A better exercise would be to go through some of your old essays and see if you can pick out any words you're repeating, or any simplistic vocab that doesn't help communicate your ideas fully. These are the ones you can look up in a thesaurus. From here you can develop your own mini-thesaurus, or at least vocab list, to refer back to. I'd highly recommend having one of these for your English T.R. texts for all the major themes or character descriptors (eg. if you were studying A Christmas Carol, rather than having to call Scrooge 'greedy' in every essay, you could use avaristic, misanthropic, usurious etc.)
Alternatively, if you're just looking to casually browse (which is more fun, but probably won't help as far as memory retention goes) then looking up random things on thesaurus.com can be a good idea. Since it's all hyperlinked you don't have to worry about looking up entries by flicking pages, you can just click your way around and drift from 'magnanimous' to 'avuncular,' kind of like that wikipedia challenge where you go from one article to a seemingly unrelated one in 10 moves or whatever.
A brilliant book to read: 'Reading the OED' by Ammon Shea <-- some guy who read the whole OED. All 27 volumes or something crazy, and the documented all the interesting words he found in this book. Very few of these will be relevant from a VCE English perspective, though it's really fun to have them at your disposal.
My personal favourite: 'peristeronic' /adj/ of or pertaining to pigeons.
Any day now that's going to come in handy...