With all the arguments now about newspaper sales, I though this article is an interesting study that shows that the current methods used in measuring Australians online numbers are also exaggerated by about a factor of three. If so this is much more an exaggeration than newspaper circulations.
You can find the study here.
I think the Australian figures online would be further exaggerated because although the study does mention it, it goes into no details of the problem of people having multiple machines. For example I use at work a computer, at home two computers, I also have an iPad and a Android Phone. That is five computer that go though three internet suppliers. Based on there study, I might be counted as fifteen people.