The gold used has to be mined from American sources. I suppose that the Mint pays them in USD and Goldmark has a source for US-mined gold and ships them the blanks. Remember that the Perth Mint (Australia's national mint) makes a lot of gold coins as well.
My comment was sarcasm. I was not seriously asking how this works, but rather pointing out the silliness of an Australian company trading a hard asset for some more worthless American paper.