Delays to a new cervical screening programme could put lives at risk, a union says.

The programme was due to switch to a mostly computerised biochemist test sometime this year, requiring fewer people to run it. However, it has been delayed another three years.

Association of Professional and Executive Employees national secretary Deborah Powell said many laboratory professionals who checked cervical cells, known as cytologists, had already started resigning to retrain or leaving the profession - putting stress on the sector.

"At risk is our ability to continue to provide women with the security of knowing they are disease free, or that they will get treated in the timely and lifesaving manner," she said.

"We're not sure what's going to happen to that workforce - it could be a disaster."

The workforce for the present programme is 120 to 150 people, some part-time, but only a fifth of those will be required in the future.

Training new staff had stopped, Dr Powell said.

Delays meant there weren't enough staff to continue the present programme and more resources were needed to ensure women received proper treatment, she said.

"Now, we are going to have to hang on for a few more years."

Ministry of Health clinical director for the screening programme Jane O'Hallahan said the ministry had reviewed the implementation timeframe for the new system and decided on a phased approach.

"For human papillomavirus primary screening to be safely introduced, the clinical pathways must be supported by a fit for purpose IT solution.

"Simply adapting the current IT system is not a viable option."

The ministry was working with the sector to make sure the existing programme was effective while it rolled out the new programme, Dr O'Hallahan said.

It was regularly meeting with the sector to manage the workforce and ensure it was well-resourced.

New Zealand had one of the most successful cervical screening programmes in the world, Dr O'Hallahan said.

The new programme will stop screening women between 20 and 25 years old from 2019.

Since the inception of the screening programme in 1990, there had been no reduction in cervical cancer incidence and mortality rates for women under 25 years of age, Dr O'Hallahan said.