The health minister has confirmed some cancer treatments have been delayed by up to seven days at Starship Children's Hospital in Auckland.

Earlier on Thursday, Health New Zealand's Starship Hospital director, Dr Greg Williams said the paediatric haematology/oncology service has had to reschedule some treatments due to a shortage of nurses.

"These decisions are not made lightly and are based on clinical need. Starship is recruiting experienced and new graduate nurses as quickly as possible," Dr Williams said.

A spokesperson for Health Minister Simeon Brown told RNZ he had asked Health NZ chief executive Dale Bramley for an explanation.

The spokesperson said all children at Starship Hospital were still being treated, though a handful of chemotherapy sessions had been delayed for a few days - but no more than seven.

Two bone marrow transplants were moved "slightly", but both were still going ahead.

The spokesperson said vacancies at Health NZ were the result of natural attrition such as parental leave and retirement, and there was no hiring freeze.

Dr Williams said the oncology departments were being supported by staff in other parts of Starship and Auckland City Hospital.

Though the spokesperson for Brown's office stressed no treatments were suspended, only rescheduled, Nurses Organisation Kaiwhakahaere Kerri Nuku was unconvinced.

"Health Minister Simeon Brown can argue oncology services are being postponed and not suspended but such semantics are unlikely to be important to whānau with children with cancer," she said in a statement.

"The haematology/oncology ward 27B at Starship has been under-staffed all year and Te Whatu Ora should have hired more nurses before they were forced to suspend vital cancer treatments for sick children."

Health NZ data obtained by the NZ Nurses Organisation showed 85% of shifts at Starship's Haemotology/Oncology Ward were understaffed during August.

The number of "shifts below target" had steadily risen since January, when 18% of shifts were understaffed, up to 53% in July, before spiking in August.

Nuku said the short staffing was "deliberate."

"This is equivalent to 6 full-time nurses short in the haematology and oncology inpatient ward for August."

A nurse had anonymously told The New Zealand Herald she and other staff were "horrified and appalled" by the news, which had the potential to adversely affect patient outcomes.

She believed parents would be shocked by the development.

"You're increasing the risk of complications and death from cancers," the nurse told the Herald.

RNZ has asked how many patients have been affected by the delays and how many staff the hospital is short of. Health New Zealand is yet to answer those questions.

The nurse also told the Herald that the severe staffing shortage was not only related to the winter flu spike but had been critical "all of this year".

"Over the last year with budget constraints, they've not hired new nurses. Anytime a nurse left, they weren't replaced."

The Herald also reported the nurse said the hospital had lost a lot of senior nurses.

Tertiary providers were recently told to limit nursing enrolments to 2025 levels, because there are not enough student placement spots available in hospitals.

Nurse educators told RNZ on Wednesday the Tertiary Education Commission (TEC) directive makes no sense as pressure on the workforce builds.

Tertiary Education Commission Chief executive Tim Fowler said told Morning Report on Thursday the limit would mean around 700 fewer enrolments.

He said enrolments in nursing had gone up by 30% in the last six years - from about 8000 to just over 11,000.