The link didn't work for me, but if I remember correctly, there was an auction for this dinner? (Please correct me if I'm wrong)
To me, they have bought time with the PM, to talk about whatever they wish. This is still a "work function", not a coffee with friends. They have bought time with the "Prime Minister of Australia", not Julia Gillard, cat lover and knitting enthusist (or whatever she may do in her own time!) Politicians wouldn't be on many people's "top interesting people" list, so most people would have an agenda.
I think it's perfectly acceptable to hold a heated discussion. The PM has been thoroughly drilled in appropriate answers & bean dipping for these types of questions. I don't think it's appropriate to rapid fire questions and attack the PM personally.