Collective Responsibility -v- Individual Responsibility.

[Aliantha]

Sometimes politicians 'cross the floor' because they simply disagree with the party line. It happened in federal parliament here a while ago. Here's a snip from an article about it.

Nationals Senator Barnaby Joyce has crossed the floor to defeat changes the Government wanted to make to the Trade Practices Act.

The Queensland Senator voted with the Opposition and minor parties twice to defeat the Government's move to limit the Australian Competition and Consumer Commission's (ACCC) power to approve company mergers.

Senator Joyce says the changes would have led to greater centralisation, and he had no choice but to cross the floor.

"The alternative was to vote for a piece of legislation which specifically spoke against what we campaigned for in Queensland, and what I'd stated in my maiden speech," he said.

And again here (from wiki for want of a better source)

Joyce also said that he would not support the Government's "Voluntary Student Unionism" Bill banning the levying of compulsory service or amenity fees by universities without amendment because he believed it would unfairly disadvantage regional universities. However, Joyce was unsuccessful in his attempt to amend the bill, and subsequently crossed the floor on 9 December 2005 to oppose it. This was ultimately futile as the Government had secured the vote of Family First Senator Steve Fielding.

He still holds office.

I believe if your constituents have voted you in for a particular reason, then you should always support their views, not the views of your party. Ideally, if you demonstrate your ability to do so as a member of a party, you then have the option of a very strong career as an independant candidate (if you have that option there).