Ankh-Morpork seems to be hard to get in Australia - last place online that has stock is Games Paradise:
http://www.gamesparadise.com.au/discworld-ankh-morpork (although friends have had terrible experiences with them in the past). Do you have a Games World anywhere nearby? They tend to have decent stock of lots of games, you may just be lucky if they have copies left.
(BTW: Use
www.boardgamesearch.com.au to look at online board game purchases in Australia, it's quite a good engine)
Guards! Guards! has a very complicated ruleset, and the game itself takes about 4 hours to play. It's also got a lot of "take that" mechanisms, i.e. you have to be mean to other players (which of course, makes the game even longer). I found it very hard to learn from the (original) rules, and tedious to play. The 2nd edition rules went some way to fixing this, but it's still a long and complex game.
The Witches is fine, it's just... weak, and a bit boring. The components are good quality, the art is superb, and the rules aren't terribly difficult (although once again, the rule book could have been written more clearly), but playing the game isn't all that exciting and it just kind of fizzles out at the end. Playing it cooperatively, it's either "Oh, we won." or "Oh, we lost", rather than anything *exciting* about the finish. (e.g. Pandemic usually has a nail-biting finish. It's a narrow win or loss.)
The advantage of The Witches is it's still quite readily available.
However, all of the Discworld board games are going to be going out of print shortly - after Terry's death, the licenses were cancelled by Terry's estate, so Martin Wallace's third game (The Gods) will never be printed[1], and there won't be any more print runs of the other 3 games[2] (probably. It's possibly Clacks might get another print run because it's so new).
So if you actually want any of the games, I'd suggest trying to get hold of them now, before they're all gone.
[1] I saw a playtest copy at Nullus Anxietas V but never got to actually play a game.
[2] Thud is in a different category and doesn't count here. Its mass-market print run was done in the 90s and has been out of print for a while, but you can still get "travel" copies from the Emporium, and I suspect they'll still be able to produce copies of Thud for as long as they like.
