Rankings in Australia are a bit odd. Unless you're a tour pro, you only really need a ranking for 3 reasons:
1) getting admission to Futures/top domestic money tournaments
2) getting admission to junior tournaments
3) getting a scholarship to play college tennis in the US
Players in category (1) make up around the top 150/200.
Below that, the rankings are virtually junior rankings - if you download the rankings list you will see that 95% of players from 200 onwards are born in the mid-90s or later.
There is a high attrition rate from category 2/3 to category 1, basically because once you get to 19/20 the top few % have got their scholarships or started inroads to a professional career. The vast majority of players are now finished juniors and no longer need a ranking. I mean, you can play plenty of money tournaments as an unranked player. Most people just drop out of the system at that point - it doesn't mean they're suddenly not good enough to be ranked. In fact (as people do) most of them get better into their 20s.
So - short answer, the top 150 players in Australia are probably the 'genuine' top 150. After that, most good adult players in Australia aren't in the ranking system so you are mostly competing against kids. You will therefore find it much easier going.
Personally I would hazard a guess that if you want to make top 200 in Australia, you need to be capable of winning qualifying matches at Futures tournaments - because that is around the level that most of those guys are competing.