The Federal Opposition is set to clinch a Senate victory on its bill to block the Fair Work Commission's (FWC) recent penalty rates decision, in a move that will put further pressure on Prime Minister Malcolm Turnbull to intervene.

One Nation, the Nick Xenophon Team and crossbench senator Derryn Hinch reversed their positions and will now back Labor's bill, which will almost certainly be defeated when it returns to the Lower House.

It comes as the Senate last night passed, as a matter of urgency, a motion condemning the Prime Minister's, "lack of empathy for Australian workers who rely on penalty rates to make ends meet".

During that debate, Senator Hinch openly acknowledged public pressure and a "very clever campaign" run by Labor had forced him to change his long-held position on penalty rates.

"I believe they should be the same as Saturday rates … on this I supported the umpire's decision," he said.

"But in my maiden speech I promised I would listen. I have decided now to go to the third umpire, and the third umpire's decision is that cuts to Sunday penalty rates are O-U-T, out!"

One Nation Leader Pauline Hanson also supported the FWC decision, arguing it would give "struggling small businesses" a chance at growth, but she too appeared to have caved in to public pressure.

Labor's employment spokesman Brendan O'Connor welcomed the apparent change of heart but questioned whether the crossbenchers were in fact trying to save their own jobs.

"I question their motives," he said.

"I think they're more interested in their own futures, not the futures of those 700,000 workers and their families who'll be so badly affected if this decision is allowed to stand."

Last month's decision by the FWC to reduce Sunday penalty rates for some workers in the hospitality, retail and fast-food industries sparked a political furore and opened up a new line of attack for the Opposition.

Mr Turnbull initially tried to distance himself from the ruling but he later backed it, arguing the Government respected the independence of the FWC and was standing up for small business.

However this tactic allowed Opposition Leader Bill Shorten to draw the battle lines between the Labor Party — which stands up for workers — and the Coalition — which backs business.

Mr Shorten warned the penalty rate cuts were the "thin end of the wedge" for workers in other industries and called on Mr Turnbull to support Labor's bill to, "protect the take-home pay of Australian battlers".

While Labor's bill to overrule the FWC's decision is expected to pass the Senate, it will almost certainly be defeated in the Lower House where the Government holds a slim majority.