The backpacker tax reversal has infuriated farmers and humiliated the government.

Treasurer Scott Morrison had been so certain of the agreement that he paraded the crossbench deal with Pauline Hanson's One Nation on Tuesday, telling the Opposition it could "go take a flying leap," because its support in the Senate was no longer needed.

However, that was before renegade One Nation senator Rodney Culleton broke ranks with his party to vote with Labor's proposed 10.5 per cent alternative, as did another senator, Victorian independent, Derryn Hinch.

That was ironic because its win on the Howard-era Australian Building and Construction Commission, by 36 votes to 33, had been sealed earlier in the day via a personal handshake between Mr Turnbull and that very same Senator Hinch.

​The Victorian rookie had opposed the retrospectivity of the original ABCC bill applying to existing enterprise agreements. Those agreements, which do not comply with the new building code, will now have a two year phase-out period.

The opposition slammed the ABCC outcome, even as its MPs conceded privately that the government had all-but gutted the watchdog, removing its most serious elements.

Mr Turnbull denied he had secured the ABCC bill - a reform pursued strongly during the Abbott period also - at the expense of its main elements, such as its strong coercive powers and denial of normal legal rights.

"We feel pretty good about it, I have to tell you. It has been a slog," Mr Turnbull said in a post-vote press conference.

Asked if his watchdog had lost its bite, Mr Turnbull's Employment Minister, Michaelia Cash, jumped in to assure Australians that the new organisation would drive fundamental change in the nation's heavily unionised building and construction sites, where the CFMEU maintains tight controls.

"Yes, it does," she said, "we have just restored penalties to what they were under the Howard government.

" As you know, one of the criticisms of the CFMEU by Federal Court judge after Federal Court judge is in relation to the imposition of this. They (employers) treat them (unions) as part of the cost of doing business . . . the strict conduct requirements that all builders now have to comply with, that also starts tomorrow when we issue a building code. So this is an important cultural reform, along with being an economic reform, and the benefits will flow to the Australian people."

The amendments were sufficient to silence previous harsh critics, with the Law Council of Australia's Stuart Clarke acknowledging that the amended bill "addresses a number of concerns held by the legal profession", including the inclusion of judicial review of any coercive powers, and the removal of retrospectivity.

"The Government, and the crossbench, have listened to a number of concerns voiced by the legal profession, and have passed legislation that will help tackle unlawful behaviour while including some important legal safeguards," he said.

Along with changes negotiated to the bill, concessions were also granted to the Nick Xenophon Team on essentially unrelated matters such as the Murray-Darling Basin Plan and an overhaul of government procurement policy to favour Australian businesses.

The 36-33 vote completed the successful passage of both union-busting bills used by Mr Turnbull to justify the early double dissolution election in July, thus meaning a joint sitting of both houses of Parliament will now not occur.

The Prime Minister praised the crossbench for backing the legislation, declaring the government had delivered on an "economy boosting" election commitment which amounted to a win for all Australian families through lower building costs, and the end to endemic lawlessness on the nation's building sites.

But Coalition relief turned to rage when crossbench senators thought to be on-board with a compromise tax rate of 15 cents in every dollar earned by backpackers, jumped ship, siding instead with the opposition.

That put the government back at square one on the backpackers tax and left Mr Turnbull warning that Labor would be blamed if the default of 32.5 cents in the dollar is allowed to come in, sending potential fruit-pickers to lower tax countries, such as New Zealand.

The Labor amendment to reduce the rate was passed by 35 votes to 32.

Ahead of the vote, Nationals leader Barnaby Joyce had gathered his entire partyroom for a press conference to declare the issue "fixed", while deputy leader Fiona Nash praised the 15 per cent rate as "a good outcome."

But it was no outcome at all.

Finance Minister Mathias Cormann immediately ruled out supporting the amended bill in the lower house.

"The people who got stuffed today by Labor, Greens, Senator Hinch, Senator Culleton are the farmers of Australia," he said.

But Labor and parts of the crossbench are now appealing to disgruntled Coalition MPs to cross the floor and back the amended bill.

Labor's spokesman for regional affairs, Joel Fitzgibbon, singled out regional electorate MPs such as George Christensen, Andrew Broad and Ann Sudmalis, calling on them to "do the right thing by their farming communities and come with us".

Senator Hinch told Sky News he decided to back the 10.5 per cent rate partly after a conversation he had at a prime ministerial party at The Lodge on Sunday when he heard Mr Morrison was opposed to the 15 per cent rate, but then embraced it the next day.

With Michael Koziol and Fergus Hunter

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