Actually if their support was that concentrated, they'd do significantly better than they do in the lower House. It's much much better to have ~30% of the population in 4 inner city seats across Australia (assuming Labor preferences flower to the Greens) than their current situation. The Nationals have as many seats as they do because their vote is very concentrated in rural electorates. The Nationals have more than 9 time as many lower house seats as the Greens despite having approximately the same number in the Senate.
You are right. I probably should have said "more concentrated."
It also seems that according to today's essential poll, most voters oppose Abbott's plan to cut income tax and raise GST.