Hi Robert
According to Algaroo, a tool that tracks the level of volatility in Google AU, there doesn't look to be anything out of the ordinary in the last few days.
I jumped into MajesticSEO to have a look at their backlinks, and what struck me was that out of their top 50 links (according to Majestic), around 20 of them have been deleted on the 21st and 22nd of September.
I'm also looking at their links on a whole - there are a lot of blogroll links, which looks quite unnatural, plus a number of directory links (which aren't necessarily bad but may have been devalued). A number of these blogrolls were site wide, so the deleted ones will have resulted in a very sharp fall in the number of links.
I can only speculate as to why those links were removed - whether prompted by Google or not - but it's clear that either removing them has caused the drop, or they have been prompted to remove them by Google after finding out about their link scheme (if it was one). Similarly the massive amount of directory links could be the cause - not because they're penalised, but because they have been devalued.
Hope that helps.