For this trip it looks like you'll need to carry a library around the world with you. If a stopover is only for a few days then you could probably avoid buying a guide and just print the info you need from the net, if you're visiting a place for longer than a week (especially if you're going to multiple destinations) then get a guidebook. To avoid carrying too much buy the Thailand, Australia and NZ books before leaving home and get the Fiji and South America books in Australia or New Zealand (Australia will be cheaper than NZ).
If you are staying in hostels then the BUG guidebooks for Australia and New Zealand are the way to go. Stanfords and the Travel Bookshop will have these and most Waterstones will be able to get them in for you or you can order them online through
www.bugbooks.com.
For Thailand I would recommend the Lonely Planet (their strength is Asia) and Thailand is one of their best sellers; for South America the Footprint guide is best and for Fiji I would buy either the Lonely Planet or Moon Guide (depending which is more up-to-date). Moon is an American publisher that has a very similar focus to Lonely Planet and many of their books are better if you can get past the funny American spelling that they use :P.
While the BUG books that I recommend for Australia and New Zealand are focused exclusively for backpackers staying in hostels (if you are staying in other type of accommodation then perhaps a different book would suit better), most other guidebooks such as Rough Guides, Footprint, Lonely Planet and Moon have a much broader appeal so a lot of the more relevant budget information is edited out to make way for listings of fancy hotels that you can't afford to stay in.