I must admit that I find different election systems anorak territory, but here is a useful short guide to the impact of AV in Australia: Matilda
This guide is a bit dated, but it is brief. A more comprehensive and up to date account is to be found at: ABC
The ABC guide shows that the number of constituencies (or electorates as they are called down under) in which preferences have had to be counted is increasing. It should also be borne in mind that although Australia has third parties (setting aside the Country/National party which has generally been in alliance with the Liberals) they have not been as electorally strong as the Lib Dems in Britain - which is not to say that they could not be important in a close contest.
As a general rule, one can state that at maximum just over 10 per cent of outcomes have been changed by the redistribution of preferences, but the figure is often smaller.
Probably recent New Zealand experience is even more relevant as they switched to a mixed member system - and one gets, as one might expect, mixed reviews of its impact on Kiwi politics.