Cambo,The reason why you never had a problem is because the tax...

Cambo,

The reason why you never had a problem is because the tax system in australia is a self assessed system. No one audits your return unless you are in a targeted group, or have something seriously unusal.

MDW,

The usual treatment is first in first out. Unless you have a reasonable basis to say that the shares you sold are of a certain parcel.

Using mongombolo's example,

If you bought 300, then 700, then 1000.
You can safely sell 700 and say it's from the 2nd parcel.

hope this helps