Changes to immigration rules to help meet demand for the 5000 more construction workers needed to rebuild Canterbury have been largely welcomed by a workers' union.

However, the EPMU has also sounded a warning over the pressure such an influx of workers will put on an already stretched housing supply.

Immigration Minister Michael Woodhouse is to change immigration policy to make it easier to bring in and keep migrant workers involved in the Canterbury rebuild.

An estimated 5000 extra construction workers will be need between December last year and the peak of the rebuild in December next year.

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"We need to remove barriers to employers recruiting migrant workers when New Zealanders are not available to do the work," Mr Woodhouse said.

The new changes will extend the maximum length of Essential Skills visas for lower-skilled workers from one year to three years, and allow holders of the visas working in Canterbury to change employers without more paperwork.

An accreditation scheme for labour hire companies that employ migrant workers on the visas will also be introduced, in an attempt to stop exploitation.

EPMU, the union for workers in the Canterbury rebuild, said it generally approved of the changes.

"This gives migrant workers who are coming to a new country and participating in a hugely important rebuild project to have more job security, training, and protection from exploitation," said EPMU construction organiser Ron Angel.

"Currently, migrant workers are vulnerable to mistreatment from cowboy operators exploiting 90-day trials to make a quick buck off the backs of construction workers. This change should help those workers."

However, Mr Angel said the supply of housing was already a major problem in Christchurch and an inflow of more workers would only worsen that.

"We are also interested in the changes to accreditation for labour hire companies. There has been a massive increase in the number of these companies operating in Christchurch since the quakes.

"Their entire focus is temporary, short-term work, so it will be interesting to see how they demonstrate a commitment to training and upskilling workers."