Pauline Hanson and Jacqui Lambie have joined forces to deliver a shock defeat of the Government's signature union crackdown legislation.

Key points: Senate vote delivers a shock loss for Coalition's efforts to crack down on unions

Senate vote delivers a shock loss for Coalition's efforts to crack down on unions It failed because crossbenchers Pauline Hanson and Jacqui Lambie joined forces with Labor and the Greens

It failed because crossbenchers Pauline Hanson and Jacqui Lambie joined forces with Labor and the Greens The Government has demanded Senator Hanson explain her vote

The Government failed to gain the support of crossbenchers to pass its so-called Ensuring Integrity Bill, which Labor had dubbed "union busting" legislation.

The result prompted an immediate rebuke of Senator Hanson from the Government, which believed it had One Nation's support.

The changes would have allowed for applications to be made to the Federal Court to deregister a union or union official if conduct was deemed inappropriate.

The Government said it was crucial to ensure unions were accountable for their actions, while Labor criticised it as an attack on the union movement.

One Nation, Senator Lambie, Labor and the Greens voted against the bill.

Senior Government ministers appeared shocked when the final vote was cast.

Manager of Government Business in the Senate, Anne Ruston, reacts to the vote. ( ABC News: Luke Stephenson )

Industrial Relations Minister Christian Porter said he was "very disappointed" with the vote.

He said the Government would seek to reintroduce the bill at an undisclosed time.

"It's for Pauline Hanson to explain why she voted against a bill that met every requirement she sought through extensive consultation," he said in a statement.

"This is important reform which seeks to ensure that registered organisations obey the law. It is as simple as that."

Senator Hanson, who controls two votes in the Senate, said her party was ultimately unable to back the bill because the Government did not agree to enough of the party's amendments.

"Despite One Nation putting forward 11 amendments to the ensuring integrity bill, there was a quantity of non-amendable changes that prevented our full support," she said in a statement.

"Under no circumstance was I going to unleash their unlimited power and zero accountability on Australian unions or other registered organisations."

There were cheers from the Opposition's benches after the bill failed to pass.

Labor's Senate Leader Penny Wong welcomed the defeat of the union bills. ( ABC News: Luke Stephenson )

Immediately afterwards, Senator Lambie and Senator Hanson spoke at the back of the chamber and were joined by members of Labor.

Labor's Senate Leader Penny Wong said the Government's attempt to "tear down the trade union movement" failed because the Prime Minister has been focused on defending his Energy Minister, who is embroiled in a police investigation, instead talking to the crossbench.

"The Prime Minister has spent more [time] defending Angus Taylor than convincing the Senate of the merits of his legislation," Senator Wong said.

"The senate has not accepted the Prime Minister's attack on working people and their representatives.

"The Senate has resolved to negative this bill and that is a good thing."

Senator Lambie said she gave the Government a chance to win her vote by putting forward amendments.

"They said no. They didn't want to bother. They lost out," she said.

The loss means the Government will have to reintroduce the legislation, or seek to change its plans, if it wants to pursue a crackdown on the unions.

The bill the Government presented to the Senate included amendments from crossbenchers the Centre Alliance, which the Coalition had agreed to in return for support.

Pauline Hanson talks to Don Farrell in the Senate following the vote on the Government's union crackdown bill. ( ABC News: Luke Stephenson )

The proposed laws would have made it easier for courts to sack union officials or deregister organisations if serious breaches were identified.

The Australian Council of Trade Unions (ACTU) declared the defeat a victory for ordinary workers.

"The vote today to defeat the Ensuring Integrity Bill is a victory for all working people in this country," president Michelle O'Neil said.

"We want to acknowledge and thank the support of the Labor Party, the Greens, Jacqui Lambie and Pauline Hanson's One Nation."