useful as an ashtray on a motorbike or useful as tits on a bull - unhelpful or incompetent person or thing
as good as one man short - describing a person who is not very productive
off like a bride's nightie - leave quickly
disappear faster than a fart in a fan factory - when a large number of people leave all at once
off like a bucket of prawns in the noonday sun - farewell said on leaving
"doing a Nellie or Farnham" - frequent farewell at the end of the evening. Nellie Melba was famous for her concerts that were advertised as her farewell shows every time she left Australia or Johnny Farnham Farewell tour
more front than Myer - meaning to have more front than Myer Emporium, the department store having a huge, seven storey frontage that is almost the size of a city block.