Sydney and to a lesser extent Melbournes Grade comps will be a very high standard. All other comps in australia will but those two in particular. In Sydney, which I can speak for as I am from there, It is rare that any import gets anywhere near first team cricket. I have only ever seen a handfull of imports play first team cricket being Nick Crompton (or Compton ...whatever, hes well known in England), Tim Murtagh (or somthing?) and Ben France who played some county cricket was hobbling between 1s and 2s a few years ago.
The reason Australia is probably more competitive is it has a far more competitive structure. In England as its all about the club you play for, you can get great players playing in low divisions as they just want to play with the club. In Sydney for example its easy. You want to play serious cricket then you play Grade Cricket and even 5th Grade is a very decent standard and its not uncommon for 4th and 5th graders to go overseas and play league cricket as the overseas in England. I know for example some clubs this year had some u19 county players who were just playing and not exactly killing 3rd team cricket.
It is however weaker in other states. For example I know a 6th Grader from sydney who moved to adelaide and was playing 2nd and 3rd grade cricket over there. Message me if you need any help so you can find a club which suits you.
From any pom who comes out here I can guarantee you one thing they do all say is that they dont want to leave and they wish they lived out here.