The Liberals and Greens and of course the independents are able to step outside the party line if they really need to, when they do this it is called crossing the floor.
Quite recently Liberals have been crossing the floor to vote for climate change laws.
http://www.google.com.au/search?source=ig&hl=en&rlz=&=&q=liberal+cross+the+floor
No matter matter what the issue a Labor MP is not allowed to cross the floor unless he has been given a conscience vote. He must vote as caucus directs or lose membership.
Top Ten Things Overheard At The ANSI C Draft Committee Meetings: (5) All right, who's the wiseguy who stuck this trigraph stuff in here?