From someone who I used to know who applied to QANTAS under the '500hrs' category but got in with a few more hours than that, if it is what you want, apply when you meet the minimums to apply.
Don't necessarily wait until later when you think you have more like the minimums that they want because one of the questions you will probably get on an interview will be: "Why didn't you apply when you got 500 hours?" They are fishing to see how dedicated you are. Applying when you meet the minimums and updating as necessary will show that you are keen.