Stamps conundrum

Dear Editor,
I am currently on a personal research project and stumbled onto something very odd and I am now hoping that perhaps a historian or anyone from the general public can give an explanation. It’s a partial envelope with stamps and postmarks of Australia and British Guiana and it is indeed very odd to have two countries from the opposite ends of the globe sharing the same envelope with their respective stamps.
Yours faithfully,
T. Pemberton

20130405stamps