The phrase 'playing for sheep stations' has both a literal and ironic usage. Literally, it is used to encourage participants to play in a friendly and not too competitive manner. Playing sport or cards or a game of some sort, but not for prizes, one might say 'take it easy, we're not playing for sheep stations'. It could be used starting a game of cards or pool for example, to check whether the game would be played for money, beer, or just pride, asking 'so, are we playing for sheep stations or what?'
In typical Australian fashion, it can also be used to mean the exact opposite, because a sheep station is such a ridiculously expensive item that nobody would bet it on a game, the phrase 'we're playing for sheep stations' can also be used to mean that the game is purely for sport, and there is no bet or prize involved.