As a result it has it's own first day of issue postmark (as shown above).
For collectors wanting to order this envelope the mail order codes are:
PSE postmarked - 6676144
PSE mint - 6676134
The postmarked envelope is not listed online, and may only be available by phoning up the Philatelic Bureau.
Colleen of OzPictormarks has also advised that Darwin is holding the postmarker for those wanting to get their own prepaid envelopes cancelled.