A Bill adding minimum wage requirements for contractors will be a shambles if it passes and will only cause problems, says the Employers and Manufacturers Association (EMA).

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Labour MP David Parker's Minimum Wage Contractor Remuneration Amendment Bill is heading for its third reading, after being backed by all parties except National and Act.

It extends the minimum wage to cover contractors who may be paid less than the current threshold.

But EMA chief executive Kim Campbell said while the Bill had good intentions, it would create more problems than it tried to solve.

"Basically they'll be unable to put a fixed price into a contract, because technically the way it works when they actually quote they may be doing two or three jobs at once. On one job look it would look like they're getting the minimum wage, but actually when you add them all up they're doing quite well.

"That's the guts of it because the Bill presumes that a contractor has one client and one job and of course they very rarely do, they have many jobs and many clients. "

He said it would not work and would end in terrible problems.

"What'll happen is that contractors won't quote, so you won't be able to get a fixed price or anything. And if you do have one there's a good chance there'll be so many variables to it that people will not know what the price is. Of course the whole contracting business then goes out the window."

Mr Campbell said in a rare move for the association it had asked its 4500 members to write to MPs asking for the bill to be thrown out and he was hopeful it could be shelved.

"I think probably common sense will prevail and I'm reasonably confident there are enough people who've got some common sense in Wellington who will see this is not going to work and it'll be returned, but at this stage we can't be complacent."

The Contractor Bill is expected to have its third reading next week.