The phrase 'not happy Jan' was popularised by an Australian Telstra ad realised in 2002. It quickly entered the Aussie vernacular.
It is used when someone is pissed off at another person (who doesn't necessarily need to be called Jan) for stuffing up in a stupid way and has inconvenienced you - but is used when more annoyed than actually angry.
Argh! I can't believe that the postman hasn't delivered my package today! NOT HAPPY JAN!