Those whose business is defending the rights of workers are naturally appalled at the Government's plan to extend the 90-day employment trial period to all workplaces.

At the National Party's conference in Auckland today, Prime Minister John Key is expected to announce planned amendments to the Employment Relations Act which will allow any employer to dismiss a worker within the first 90 days without being liable to a personal grievance claim for unjustified dismissal.

When the scheme was introduced last year, as the recession was starting to bite, it applied only to workplaces with fewer than 20 employees - which, it should be remembered, is about 90 per cent of them. Its intention was to encourage employers to take on staff without fearing a protracted employment dispute if the employee proved unsuitable or the economics of the decision didn't stack up.

Unions have vowed to fight the extension and Opposition leader Phil Goff seems keen to use the issue to lubricate his party's rusty links with labour. But their case would be stronger if they could produce evidence that the scheme has resulted in a significant erosion of workers' rights.

Employers and Manufacturers Association figures show that three out of four employers have used the scheme and almost 90 per cent have kept workers on. Those aren't bad results.

Anything that encourages an employer to take a punt on a new worker - and in particular to give a chance to someone who shows promise but lacks credentials - must be worth trying. It defies common sense that cost-conscious bosses will casually sack someone they have spent three months training.

Foreshadowed restrictions in union access to workplaces are more disturbing. There is no evidence that the present system is not working, and the amendment as proposed will make it harder for union representatives to protect their members' rights and easier for bad employers to escape scrutiny.

That is in no one's legitimate interest. But in opposing the extension of the trial scheme unions seem more driven by ideology than good sense.