The Health Ministry has said Waikato Hospital's Henry Rongomau Bennett Centre is not fit for purpose (file photo).

Waikato's burgeoning prison population will have a dramatic influence on the region's mental health services, health bosses say.

Waikato DHB plans to replace the Henry Rongomau Bennett Centre and gave an update on options at its October board meeting.

Both the Health Ministry and the DHB say the current building puts patients and public safety at risk.

Waikato DHB interim executive director for mental health and addictions Vicki Aitken said the new build was part of a wider process examining the DHB's whole model of care.

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Possible options to replace the Henry Rongomau Bennett Centre include: a new build on the main Waikato Hospital campus site, a fit-for-purpose build on a greenfield site, or a "hybrid" option with mental health services being provided at the hospital campus as well as a rural setting.

Aitken said a key consideration for the DHB was the proposed expansion of Waikeria Prison from 740 to 3000 beds.

"We've been very clear that this business case is wider than just our acute inpatient facility. It does need to take into account the fact we're going to have more prison beds on our doorstep," she said.

"There will be a massive impact, not just on the forensic services, but adult mental health services [and] addiction services."

Derek Wright, Waikato DHB interim chief executive, said the Health Ministry and Treasury had been intimately involved in the business case process.

"At the moment, they're heavily engaged with us, which is why it [the business case] has probably got more elements to it now," Wright said.

Aitken said staff were continuing to hold workshops to discuss the new build options.

The DHB has previously said it hopes to open the new complex within five years.