A lobby group believes taxpayers should be shocked that an electorate MP can simply quit when they get a better job offer, six months into doing what they were elected to do.

The Taxpayers' Union is spitting fire at Jonathan Coleman's decision to resign from Parliament to run a private healthcare company.

Executive Director Jordan Williams says when you stand for Parliament it's not an "all or nothing" thing.

"The National Party didn't get back into government, but that's not a reason to up and leave the electorate of Northcote and leave taxpayers with a three quarters of a million dollar bill for a by-election."

Williams says surely as an electorate MP Dr Coleman should be required to serve out the term.

"It's really about honour. You stand for parliament, you're standing for a three year job. It's effectively a breach of contract, or an implied contract."