The Group of 8 isn't based on rankings, as many people have already replied and told you.
The universities are also not the "oldest" in Australia, although many of them are older.
They are a group of research intensive universities. I'm not saying any of them are bad universities, but research intensive does not automatically equal good teaching, the subjects you want etc. All that can really be said of research intensive universities is they tend to attract more accomplished lecturers.
I'm doing a "lowly arts degree", and I enjoy it. I didn't do great at year 12, but I got into ANU. And if I would have achieved a 90 or above UAI I would still do an Arts degree. Its what I want to do, I would rather not go to university, than go and do an economics or maths degree. Sure, there are probably more options to get higher paid jobs after graduation but I wouldn't want those jobs. Some people's priority is money, and while I'm not saying that I don't want money, I just would rather do something I enjoyed than earn lots of money. When people start bashing other people's career choices they are just showing how they don't understand that some people think differently to them. It's okay, they will learn eventually.
Anyway, back to universities. I believe (going of memory here) that in this year's good university guide the uni's that ranked highest for teaching (from perspective of graduates I think) were not Go8 universities. In fact, they often don't really rank that high in a lot of things. I have heard countless times people complaining about crowding and lack of english skills at universities like Sydney, and people over all not being so happy.
Just because a lecturer is accomplished does not mean that they are a good teacher. There are a lot of lecturers that just don't do it for me, just can't lock on, concentrate, relate. I'd much rather a lecturer that isn't so accomplished but can explain things so I understand, and provide thought provoking problems.
I'm applying for a transfer to a non Go8 university. I don't think any of the Go8's offer the lifestyle that I want, and for an Arts degree it doesn't really matter.
If I were to do post grad I would probably limit my applications to Go8. Yes, some American Universities are better. They have more people. Their top 10% will be equal to our top 0.5%. If you can afford it, by all means, go to America. Honestly though, most people won't really need that kind of education to live the life they are going to live.
Anyway, just my opinion on things.