There is only one person that I would recommend and that would be Bill Weise at
http://www.stampexpertizing.com/. There are many other groups that certify stamps such as the PF, APS and others. You can read about Bill here or on VSC.
As to who to do it down under, I haven't a clue.
However, what you have may be nothing more than a smear on the printing plate, that's why I asked about duplicates. If you can determine that it is a consistent, repeatable defect then your chance of getting a minor variety becomes much greater.
Art