THE SENATE has become a rabble because of what a senior Labor figure calls the betrayal of voters by members switching parties.

The latest party jumper is Tasmanian Steve Martin, who today confirmed he has joined the Nationals.

Dubbed the "Tasmanian Tiger" by his new colleagues - because Tasmanian Nats were thought to be extinct - he has changed his political stripes three times since arriving in the Upper House in February.

Senator Martin was elevated to represent the Jacqui Lambie Team after the party's founder was forced out by a dual citizenship problem. He then became an independent, and is now in the Coalition as a National.

He is one of five senators who came to the Upper House representing a party but subsequently dumped it.

The others are:

Lucy Gichuhi, who was elevated to replace Family First's Bob Day but became a Liberal. She had little choice as Family First effectively disappeared;

Cory Bernardi, elected as a Liberal but quit to join his own Australian Conservative Party;

Fraser Anning, who was elevated to replace One Nation's Malcolm Roberts, but had a row with Pauline Hanson before taking his seat and entered parliament as an independent;

Tim Storer, elevated to replace a Nick Xenophon Team senator but entered the Senate as an independent.

An earlier example of party dumping was One Nation's Rod Culleton, who was ousted from parliament on constitutional grounds and spent his final parliamentary days as an independent.

This tally is in addition to the 15 senators who have left parliament because of constitutional breaches or voluntarily since the 2016 election.

The prominent political consequence has been instability on the big Senate crossbench, making the passage and blocking of legislation difficult for the major parties to manage.

Labor frontbencher Brendan O'Connor today also called the switching "a sign of betrayal" of voters who could have expected the senators to represent them through their original political party.

"There are now up to six senators who have started and been elected by the constituency in one place and have moved elsewhere," Mr O'Connor told Sky News.

"It does speak to the instability of the cross bench … It is extraordinary that so many now have actually moved away from where they originally were.

"So they've told the electorate one thing and they've made a decision subsequently to join someone else. In a way that's a sign of betrayal."

He said these big steps were happening "with such frequency at the moment".

"I think people have a good reason to say, Well what's happening with the cross bench?"