The mother of a toddler whose cancer treatment has been delayed due to staff shortages at Starship Hospital says she's heartbroken, and doubts the health minister's assurances the jobs are being recruited for.

Some chemotherapy and bone marrow treatments have been delayed at the children's hospital for up to seven days due to lack of staff.

Tanya Lapu'aho's two-and-a-half year old son was diagnosed in January with high-risk neuroblastoma.

He's since had multiple stays at Starship for rounds of chemotherapy and two bone marrow transplants, and is moving on to radiation and immunotherapy.

Lapu'aho said timing is everything with cancer treatment - but her son's second bone marrow transplant was delayed at least a week because there were not enough specialist nurses to carry out the treatment.

She said babies were bearing the brunt of delays.

Lapu'aho's son had to wait for chemotherapy lines to be changed and medication to be administered, and said available nurses would apologise because they were not yet qualified for those tasks.

She said staff were overworked and under-resourced, and a charge nurse had told her there was a hiring freeze.

Health Minister Simeon Brown insisted all eight vacant paediatric haematology/oncology roles had been filled or recruited for.

Health NZ since confirmed it would be three months before the ward was fully staffed, with seven nurses due to start over that period.

A spokesperson in Brown's office denied any hiring freeze and said the vacancies were the result of natural attrition such as parental leave and retirement.

But Lapu'aho said those could be planned for months in advance.

She said it was tough trying to advocate for her child because there was nothing the nurses themselves could do about the shortage.

Other parents were scared about complaining because they did not want to risk receiving poor care, Lapu'aho said.