Unions are protecting thousands of new workers from the Government's 90-day trial period law by negotiating collective contracts that exempt them from it.

But a battle looms in the public sector, where unions say they will fight a State Services Commission order that Government agencies must include the trial provision in all contracts.

The Government last year passed changes to employment law, including extending the 90-day trial to all employers and making union access to workplaces dependent on employer approval. The laws come into effect in April.

* From April, all new workers, regardless of the size of the employing company, will be able to be dismissed within a 90-day trial period and will not be able to take an unjustified-dismissal case.

* Unions have negotiated with employers including Fonterra and Victoria and Massey universities to exempt new workers from the 90-day trial.

* The Government wants all new public sector workers - including police, teachers, nurses and firefighters - to be subject to the 90-day trial, but the unions are promising a fight.

The 90-day trial, which prevents newly hired workers taking a personal grievance case for unjustified dismissal, is now available only to employers with fewer than 20 workers.

But unions have been negotiating to exempt their members from the new law by keeping it out of their collective contracts.

Agreements signed include the Dairy Workers Union with Fonterra, the Tertiary Education Union with Victoria and Massey universities, and the Service and Food Workers Union with the TAB and some aged care and residential homes companies.

Last month the State Services Commission told Government agencies - among them the police, the Fire Service, and the health and education sectors including teachers and doctors - that it expected the 90-day trial and union-access rules to apply.

"The changes are Government policy. Accordingly, state sector agencies are expected to implement them," wrote the deputy commissioner of state sector performance, Peter Brown, in an email to Government agencies last year.

Agencies should not "contract out of aspects of the new provisions or contract to restrict any application of them", said the email, obtained by the Herald.

Service and Food Workers Union national secretary John Ryall said district health boards were feeling the pressure.

"We've got 3000 members working in public hospitals. DHBs told us they don't want to change the status quo, but the Government expects them to implement Government policy. We've told them that's not acceptable."

Public Service Association national secretary Brenda Pilott said employers were usually free to offer better conditions than the statutory minimum.

"The commission has taken a very hard line on this. They're saying that you must have these legislative minimums in your contracts. It's tying the hands of state sector employers."

If the employers tried to put the changes into contracts coming up for negotiation, "there will certainly be some stoushes".