I think that any Parliamentarian at either state or federal level (even council level) who is elected as a member of a political party (eg Liberals, Nationals, Labor, Greens, etc) who at any stage during their term in office chooses - for whatever reason - to leave that party and sit as an independent should automatically trigger a by-election in that seat by doing so.

Why? Because while a majority of the people in that electorate may have voted for that individual, they did so on the understanding that they supported the platform of the party of which they were a member. By sitting as an independent after being elected, they deprive the people of a representative who aligns with the wishes/ideals of the voters who got them there. It also deprives that party of a member after they put time and money into winning that seat. What are your thoughts on this?

No - we shouldn't have to pay for another election.

Not all party members agree with every policy -
sometimes real policies are hidden from the voters before elections.