1st National I went to was in Sutherland, southern suburbs of Sydney, very, very wet. This may be the one Rowan is referring to, but it was 1987 or 1988.

I've also been to one or two that were very wet in Canberra, but usually the only weather problem is the temperature difference between 1 pm and 11 pm. However you dress you'll be uncomfortable part of the day, or you'll be lugging warm clothing around, or you'll have to go back to the car/tent/whatever to dump or pick up stuff.

It's a small price to pay for a terrific festival.