Well it has to do with the kind of pitches we prepare at home. Even a top flight batsman like Sachin has only one (out of 42) hundred in Australia in 20 odd matches
On a positive note Pakistan's only WC victory to date was in Australia and in that tournament we did beat Australia in the pool game :)
And we also won the 1996/97 triangular series, in which we beat Australia at least a couple of times. In 2002 Pakistan won the three match series against Australia. We've beaten Australia numerous times in ODIs in Australia, but hardly ever in a Test. One time we got close to winning the Test series under Imran in the late eighties, but a typhoon caused loss of a few playing days which let Australia off the hook.