Back from New York

I'm back from 6 days in New York, and today I've sent off my passport so they can put the visa sticker in it. I've also started reading through the raft of documentation and guides I've been issued to help me settle in Australia. My favourite so far is this from the Australian Customs and Law section, under a heading of 'Common Australian Expressions'. There are just three listed:

Bring A Plate - this means bring a dish of food to share with other people

B.Y.O - this means to bring your own drink

Fortnight - a fortnight is a two-week period. Many Australians recieve salary or wages every fortnight.

Of all the three things Aussie expressions...why the hell those ones?