Around the start of the new year Google did it's math wrong when they erroneously calculated AdSense remuneration for publishers from Australia and New Zealand.
As a consequence of applying inverted the exchange rates to calculate payments, the poor publishers ended up getting 20% less than what they should have been paid.
Fortunately for the Australian publishers, Google's acknowledged the mistake and has promised to reimburse the publishers their due share. Mails have been forwarded to all the publishers affected by this issue. The issue pertains to the November payments for publishers.
Google's offered an apology and an assurance "We'd like to apologise for the inconvenience this has caused you, and thank you for your patience as we work to issue the additional payment due to you. This payment will be issued early next week."
In case you haven't got a mail from them you can mail them here