The Southern District Health Board said the investigation should be complete next week.
Alarmingly, a similar error had already occurred at the Waikato Hospital Laboratory 18 months before when a woman's tissue samples were placed in the wrong processing cassette.
Taupo resident Jenny Engels, 65, was wrongly diagnosed with cancer and, as a result, had her left breast removed. Four weeks after the invasive surgery she was told about the devastating error.
In a letter to Engels, the Waikato District Health Board said it would make changes, including eliminating the batch processing of samples and deploying staff to do cross-checks.
"They said they had put things into place like double- check specimens. Well, I would have thought that is what they should have done in the first place," Engels said.
"They are not small health boards, they are big concerns and, if it can happen there, it can happen anywhere if they are not [careful].
"I want improved protocol for diagnosis so this error does not occur again."
Health Quality and Safety Commission spokeswoman Liz Price said 377 people were involved in serious events that were potentially preventable during the past two years.
Clinical management events, including errors of diagnosis and treatment, accounted for 29 per cent of the incidents.
Price said more work needed to be done to ensure that lessons learned and recommendations from these types of mistakes were implemented.
