Cash-strapped police face a $20 million bill for technology to support the government's new child sex offenders' register.

The database is expected to be up and running next year, but $61m is needed for new technology and development over a decade. However, documents reveal cabinet has so far only approved $35.5m.

A new child sex offender registry is being set up in New Zealand. Photo: File.

A regulatory impact statement (RIS) indicates police will need to find $19.6m, with corrections, courts, ACC, or other government agencies, responsible for the rest.

However, social development minister Anne Tolley says the technology is still being developed and 'police will not be expected to bear all of the costs.”

The proposed register will keep track of child sex offenders aged 18 and over. It's expected that 470 offenders will be registered in the first year, rising to almost 3000 within a decade. The most serious offenders will be monitored for life.

The index – which will contain fingerprints, photographs, aliases, addresses, workplaces and employers, car registrations, IP addresses and passport details – won't be open to the public and will be maintained by police and corrections.

It is expected that the total cost of setting up and managing the register over ten years will be $146m.

Corrections will stump up the largest of the set-up and operating costs (around $70.6m), but this will be absorbed because they are already manage high-risk sex offenders. An extra $9.1m in 'additional unfunded costs” will come from 'efficiency gains”.

Police expect their $14.1m contribution will also come from savings made through more effective deployment of officers. The force has faced a funding squeeze over successive years, with the 2015/16 budget dropping by $15m.

However, when it comes to investment in new technology, the RIS notes: 'Agencies are unable to absorb the technology, ongoing operation and evaluation components of the register.”

Police policy analysts were unable to carry out a cost-benefit analysis, but estimate the register will prevent between 4-34 offences.

And because up to 90 per cent of sex crimes are not reported to police 'it can be assumed that some of these unreported instances of offending would be prevented.”

Tolley has not ruled out more cash for the register.

'We work on four year budget plans, and $35.5 million has already been committed to the technology element,” said a spokesperson.

'Additional funding will come from a mixture of future budgets, any savings from expenditure reviews of agencies, and contributions from other agencies involved in the operation of the register.”