That would be true if everything in Oz was "twice" the price, but it's not. Average house price in Canberra is about £200k, average price in London is £350k. And the figures for London will be substantially less than they were in June 2007 when the Oz ones were measured.
Therefore they do have much greater buying power.
Median house price figures aren't very helpful in this context because they don't tell the full story. In any case, that figure of £200k is only for units; not houses (and pretty damn small units, at that). Median price for a house in Canberra is actually £309k.
If you want to compare buying power it makes more sense to compare the price of goods and services that both sets of MPs are purchasing, like food and temporary accommodation. And as plenty of ex-pat Brits will tell you, the average cost of living is more expensive in Australia than the UK.
At any rate, this all begs the question "If Canberra prices are so cheap, why don't Australian MPs buy second homes in Canberra for the purpose of attending Parliament House?" Most likely for the simple reason that they don't need them (and probably can't afford them anyway). Yet British MPs seem to have no trouble buying second homes in London. And why do they need them in the first place? No reason that I can see.
Let's also remember that some MPs are lazing around in "grace & favour" homes (for which Australia has no equivalent). Don't even get me started on those.
