The Coalition's proposal for a 19 per cent tax on all backpackers' earnings is in disarray.

The bills look set to fail in the Senate as Labor, the Greens and crossbench senators harden their opposition to the plan.

Labor and Tasmanian independent senator Jacqui Lambie will each move an amendment to set backpacker tax rates at 10.5 per cent, in line with New Zealand.

Shadow Treasurer Chris Bowen said he was confident that would be endorsed by the Senate, sending the amended bills back to the House of Representatives.

"National Party MPs will have the choice — do they support 19, or do they support 10.5?" he said.

"And they need to explain to their electorates, if they vote for 19, why they do."

The Greens will propose changes that would see backpackers taxed at the same rate as Australians, and which would allow anyone to claim the $18,200 tax free threshold, regardless of whether they are Australian residents.

Senator Peter Whish-Wilson confirmed if that amendment failed, the Greens would support an amendment for a 10.5 per cent backpacker tax.

"Then, if the Government chooses stupidity over sense by not supporting the amended bill, the bill will be dead and the Government will be the ones who killed it," he said in a statement.

Higher tax rate will kick in if bill does not pass, Joyce says

The National Farmers' Federation (NFF) is now seriously concerned the Government will refuse to bring the legislation forward for a vote, leaving the agriculture sector in limbo.

Deputy Prime Minister Barnaby Joyce is adamant the Government will enforce a 32.5 per cent tax on all backpackers from January 1 if the 19 per cent compromise rate does not pass the upper house.

"That is just the fact of the legislation," he told reporters.

"The Administrative Appeals Tribunal came through [in March 2015] and said 'We believe that these people have been claiming the tax-free threshold and they shouldn't'.

"We had to react to their ruling, and their ruling will remain in place as of the 1st of January next year because of the way the Labor Party are playing this, and that is a disgrace."

Tax stays the same unless Parliament sets new rate

But it remains unclear whether that is in fact the case.

Parliamentary Library advice provided to Senator Whish-Wilson indicates the status quo will remain unless Parliament passes legislation that sets new tax rates.

Non-resident backpackers would only pay the 32.5 per cent rate if the Australian Taxation Office cracked down and pursued them for tax avoidance, the advice suggests.

That is likely to be of little comfort for Australian farmers who rely on backpackers to pick and pack their crops each year.

For them, the damage of the past 18 months of uncertainty and politicking has already been done.

Farmers in Tasmania have already reported a 40 per cent drop in the number of backpackers seeking work.

More uncertainty will cripple agriculture: NFF

NFF acting chief executive Sarah McKinnon slammed politicians' handling of the issue, saying the ongoing uncertainty will "cripple the agriculture sector", "devastate" Australian farmers, and leave the summer harvest "to rot" unless a resolution is found soon.

"It's no wonder that the Australian people have lost faith in the political process," she told reporters at Parliament House on Tuesday.

"It's been 18 months that we've been working to get a solution. Finally at least we know what the Labor position is, but what we don't know is if this issue can be resolved at all.

"We've said all along that 19 per cent is a fair rate and a competitive rate.

"We've also said that if the Parliament will support a lower rate — and that can get through the Parliament — we'll support it."

Ms McKinnon said the NFF would not support ongoing uncertainty for the farm sector.

"Even last night I was talking to a Western Australian farmer who can't get workers, who's had to ring friends in the UK to send them out because the backpackers aren't looking for the work," she said.

"This is not a sustainable position for the sector."