With the Labor government aiming to pass the bill before the year is out, the letter blasts the timeframe on the bill as "absurdly short" and says jobs and businesses are in jeopardy if it becomes law.

It's signed by Australia's national, state and territory chambers of commerce, and calls on senators to try and split controversial elements out of the bill and pass some straightforward changes as soon as possible.

If that can't happen, they want senators to oppose the bill in its entirety.

"As we emerge from the pandemic, businesses are dealing with surging energy costs, rising interest rates and face serious global economic headwinds," the letter reads.

"The last thing we need is for jobs to be in jeopardy because of rushed new rules and red tape."

The letter outlines business concern that multi-employer bargaining changes will "drag them into a complex system they can't afford".

They also say higher costs will drive inflation and push up interest rates, and "one-size-fits-all" wages will undermine flexibility and productivity.

"We all share the ambition of wage growth, but lifting productivity is the way to do this, not by taking control away from businesses and their employees," it reads.

But union boss Sally McManus hit back, stating concessions the government had already promised the business sector would make getting minimum wage increases harder.

"We also obviously do all the heavy lifting for the minimum wage increases, and people get those increases without being union members ... it's really about giving workers options and a chance to be on a more level playing field," she told Sky News.

"We are actually concerned the concessions the government's made to the business community are going to impede our ability to get those pay increases, but getting the laws through will make a difference."

Council of Small Business Organisations Australia CEO Alexi Boyd took issue with another aspect of the bill, classifying small businesses as any entity with less than 15 employees.

She said that threshold should be raised to around 100 employees, saying a 15-person headcount was unrealistic.

"That's aligned with what other business groups are talking about, the reason for that is we need to have a conversation about what a small business looks like," she told Sky News.

"This is part of the bill that needs to be acknowledged as complicated, we need to sit down and look at what the issues are ... that's something we haven't had a chance to do."

The bill has passed the lower house and will be debated in the Senate in the next fortnight.

The government is seeking to secure the support of independent senator David Pocock and the Greens to get the bill through.

Australian Associated Press