An "internal error" caused it to claim that only two radiation therapists had been sent to the North Island recently, the agency said yesterday.
HNZ Southern admitted earlier this week six had been sent north - four to Wellington and two to Palmerston North - after other sources told the Otago Daily Times the actual figure was higher.
Interim hospital and specialist services leader Hamish Brown said HNZ Southern apologised for the confusion.
Dunedin Hospital had just three permanent radiation oncologists, and HNZ Southern was so far unable to fill a further three vacant fulltime roles.
Mr Brown yesterday rebuffed the claims of industry representatives that HNZ Southern needed to offer more pay to attract radiation oncologists.
While not specifying pay rates, he said a "competitive remuneration package" was offered.
"Even with higher locum rates and active recruitment campaigns, we are unable to secure fulltime permanent staff."
Radiation oncologists from Auckland, Waikato, Wellington, and Canterbury had helped support Southern this year.
Various radiation oncology locums included a fulltime international and three who were part-time - one providing virtual consultations and two visiting Dunedin each month.
Meanwhile HNZ Southern had sent radiation therapists to Palmerston North between June and August, and to Wellington between May and June.
The Association of Salaried Medical Specialists criticised HNZ earlier this week for offering salaries that were falling behind inflation while major staffing shortages meant more pressure for existing radiation oncologists.
Executive director Sarah Dalton said more money had to be offered as part of a solution, as there was a global market for medical skills and Australia was "vigorously" trying to recruit doctors from New Zealand.
Apex union radiation therapist advocate Omar Hamed said last week that radiation oncologists needed to be offered more to attract them to work in Dunedin.
Patients needed to see a radiation oncologist for their first specialist assessment before they could be treated, he said.
However, the shortage of radiation oncologists was causing a backlog of waitlisted patients, limiting the work radiation therapists could carry out.
The hospital had three linear accelerators - machines used by radiation therapists to target cancer tumours - but a 30% spare capacity, he said.
It was an "insane" waste of a precious resource that would not be solved by sending people north, he said.
Mr Brown said that as of August 21, there were 103 Southern patients waiting for a first specialist assessment with a radiation oncologist.
"If we were fully staffed the wait times would be shorter. However, wait times have been reduced by locums," he said.
There was was linear accelerator treatment capacity because of "a reduced throughput of patients able to be seen locally".
This did not mean patients were waiting longer compared to other parts of New Zealand, because a small number of patients had been provided the option to be treated in a private facility instead, he said.
Sending radiation therapists to other hospitals to support patients was a testament to the networks HNZ Southern had established across the national system, he said.
It was the second time in two months that HNZ Southern has apologised after providing inaccurate information.
Director for hospital and specialist services Dan Pallister-Coward apologised for any confusion after telling RNZ last month 90% of patients were getting their first appointment within a month - a figure subsequently corrected to 70%.
