Tony Abbott didn't hold back, angrily demanding that Bill Shorten either furnish proof he was not a dual British-Australian citizen, or "shut up".

Unless he has a letter, Abbott decreed, the Opposition Leader is in exactly the same position as the Deputy Prime Minister, Barnaby Joyce - ie a very weak one because, like Joyce, he would have to cravenly plead ignorance of both his personal heritage and the Australian constitution before the High Court.

A few hours later, and within minutes of Malcolm Turnbull joining Abbott's call, Bill Shorten suddenly caved - or called their bluff - tabling his letter to a gobsmacked Parliament.

The result was not good for the government. It can hardly have improved Joyce's pitiable case, nor lifted his morale.