If you are the leader of a party known for sending a cashed-up budget into the red when last in office, then the last thing you should do is behave in ways that confirms your opponents' worst depiction of you.

That's effectively what Bill Shorten has done with his "10-year economic plan", which promises bigger surpluses in the so-called medium term - 10 years from now - but admits the deficit will first go backwards over the conventional four-year budget cycle.

Malcolm Turnbull, Scott Morrison and Mathias Cormann could hardly believe their good fortune on Wednesday.