I think it's likely that the average Australian vehicle is heavier, since the average European/Australian passenger vehicle is the same age (10.x years).
Maybe our sulferous crap fuel has something to do with it too. I don't know.
Many of the cars you see on European streets would seem a bit... small... to many Australians.
And the roads in reverse. Australian roads are massive (and fast) compared to Europe.
This is true of urban areas but not expressways. Motorway limits vary from 120 (Spain) to 140 (Poland) with Germany having sections of unlimited autobahn.
Outside of the cities, this isn't true - Australian speed limits and average driving speed on highways is much lower than basically anywhere in Europe.
Inside cities, Australia definitely does have higher average speeds than inside most European cities though.