A wound in a woman's breast wouldn't heal properly because it contained a 5cm-long "foreign body".
The Auckland District Health Board has been found at fault in the case, which involved its district nursing service.
In 2013, the woman, aged 33 at the time, gave up breastfeeding her baby because of the pain in her right breast, which had a lump and was red. Her GP referred her to hospital, where an abscess was diagnosed and surgically drained.
District nurses regularly dressed the 3cm-deep wound using a variety of products, including "Aquacel rope", an absorbent material, Health and Disability Commissioner Anthony Hill says in a decision published today on the case of "Mrs A", whom he does not name.
"When packing a wound with Aquacel rope, about 1cm at the end of the rope should be left resting on the skin," Mr Hill said.
When the end of the rope was not visible it was assumed - wrongly - that the woman had removed it herself.
The wound was slow to heal and three months after the dressings began, surgical excision of the wound revealed a 5cm piece of Aquacel rope dressing and this was considered to be the cause of the failure to heal properly.
A swab from the wound earlier had produced "light growth" in the laboratory of MRSA - methicillin-resistant staphylococcus aureus.
"It is concerning, first, that Aquacel rope was left in Mrs A's wound and, secondly, that
a number of staff failed to identify the error," Mr Hill said.
"Because of the poor record-keeping and the limited assessments of Mrs A's wound by
the district nurses, it is unclear when the Aquacel rope was left in the wound, or by
Mr Hill held that the DHB had breached the code of patients' rights.
"It was the responsibility of ADHB to have in place adequate systems and oversight of staff to ensure that Mrs A received appropriate care."
The commissioner said the DHB had done a review since this case and implemented changes. He asked the DHB to make a formal written apology to the woman.