We Australians have a kind of deadpan sarcasm or a way of telling lies that looks initially like the stone cold truth. How do we recognise it in each other while it slips right by foreigners? I honestly don't know. I think part of it is shared knowledge that we are telling lies, and part of it is the occasional wink and nudge to others who share the joke, or keeping the straight face only long enough to tell the lie, then giving a warm smile, a slap on the back and a "Nah, only joking, mate". Perhaps our faces are a little too serious when we talk that way. It's really hard to pin down, but I'm sure there's some signal, or a distinct lack of signal, that points out when we are doing this.
Mokalus of Borg
PS - A search for Australian Sarcasm turned up this page of jokes as the top result.
PPS - It is not very informative, but quite amusing.