Unions are offering to go into battle for non-union members laid off under the new 90-day trial employment law.

All 42 unions affiliated to the Council of Trade Unions have agreed to take calls on the freephone number 0800 1UNION (0800 186 466) from any worker laid off during a 90-day trial period.

Callers from today will be directed to advocates from each union on a rostered basis, with all unions sharing the cost.

The CTU also launches a $42,000 advertising campaign on commercial and student radio today in a bid to reach non-unionised workers employed on trial periods.

"There won't be a union at the Four Square in Kaitaia or the service station in Waiouru or whatever," said CTU president Helen Kelly.

"The whole union movement has got behind it on the basis that these workers probably won't be in a union. A lot will be in their first two weeks of work and might not have been offered the chance to join a union even if there is one there.

"We will not stand by and see those workers get no advice."

The new law, which came into force yesterday, allows employers with fewer than 20 staff to take on new employees for trial periods of up to 90 days, and prevents any employee dismissed during a trial period from taking a personal grievance or other legal action against the employer.

The provision can only be activated where the employer and employee agree to include it in their employment contract.

Employers and Manufacturers Association chief executive Alasdair Thompson said he did not think many people would actually use it.

"I don't think too many people will be agreeing to a 90-day trial - not existing people in the workforce who are changing to another job," he said.

"Where we see it working is where a person in normal circumstances would have very little show of getting employment, an employer would say, 'Okay, I'll give you a go' - such as people coming out of jail, which is 70 a week in Auckland alone, people with no CV and no track record."

NZ Law Society employment law convener Michael Quigg said employers might also use the law to take on new employees in sectors such as information technology, where workers have often been taken on as independent contractors.

Christchurch beneficiary advocate Rebecca Occleston said workers laid off during a trial period could face stand-downs before qualifying for welfare benefits.

The law provides for a standard waiting period of a week for anyone previously earning less than the average wage, two weeks for those above the average, and 13 weeks for anyone who either left a job voluntarily or was dismissed for misconduct.