A hospital patient kept in an over-stretched secure unit is accused of raping another patient, sparking a police investigation and a major hospital review.

The incident has prompted planning for a new wing to protect vulnerable female mental health patients.

New figures show Auckland's frontline mental health emergency care facility within Auckland City Hospital, Te Whetu Tawera, is bursting at the seams.

The investigation follows a tragic series of events stemming from the parlous state of mental health care in Auckland:

The release of Mathew Ahlquist, against staff advice, who then burned and bludgeoned a man to death.

Criticism of "unsafe" release procedures after patient Shane Fisher was released then killed himself.

An independent review 2 years ago into serious and fatal incidents criticised lack of leadership at Te Whetu Tawera.

Figures released after an Official Information Act request to the Auckland District Health Board show Te Whetu Tawera is under intense pressure. It had nearly 100 per cent occupancy for every month in the past 18 months. Ideal occupancy is 85 per cent.

The figures also show patients are being kept in the emergency care facility for up to six months, when a recent national conference was told such facilities should not hold them for longer than 21 days.

The rape allegation was also revealed, although Auckland health board mental health clinical director Clive Bensemann would not offer details around the case to protect the woman's identity.

He also would not explain how a serious sexual attack could happen in a fully-staffed secure mental health facility with 58 patients as he feared it could retraumatise the woman.

However, he said the allegation led to a new policy being developed by the health board on how "sexual intimacy" among patients should be handled. The policy also included dealing with sexual assault and rape complaints.

The health board said the woman received medical help and counselling. Police were called and an investigation launched.

The complainant and alleged attacker were placed into different wards and planning began for a "vulnerable women's wing" on one ward, expected to be available later this year.

Bensemann admitted that patients at the unit have few places to go because the rest of the system is overloaded.

He said the decisions about priority were made by clinical staff and at times Te Whetu Tawera admitted seriously unwell new patients by shifting out others who were also sick, but not as sick.

Those people were moved into residential mental health facilities or into the community.

However, others were so unwell they were kept in Te Whetu Tawera long beyond the time they should be because there was nowhere else to go.

Those patients with "high and complex needs" tended to be retained for longest. Those patients needed - but had trouble accessing - acute care with rehabilitation programmes, which were not available at Te Whetu Tawera.

He said occasionally other health boards were approached to ask if they could take patients but they were similarly stretched. "It's true that mental health operates with constrained resource like other parts of the health service.

"Delivery of care tends to include prioritisation."

Bensemann also said that Te Whetu Tawera had to rely on staff willing to do overtime, and using temporary nursing staff.

That overtime, welcomed by some staff, could be "quite stressful" for others.