The DHB spent $597,000 more on medical staff than was budgeted, and $1,584,000 more than budget so far this financial year, senior business analyst Bron Anderson's report said.
In what was known as the "provider" result, January still delivered a $605,000 surplus, but the board budgeted for a $2,117,000 surplus. The year-to-date position was $912,000 worse than expected.
Staff were still confident of meeting the budgeted overall provider-arm deficit of $491,000.
Mr Butterfield was perturbed by the overrun, saying "I've lost $1.5 million of what I thought I would have".
"Dare I call it a blowout?"
Describing the result as "a bit of a shock", Mr Butterfield questioned how staff could be confident it was a one-off.
"That does not gel."
He did not want to delve into the "industrial" specifics relating to the personnel spending in the public setting of a meeting.
"I just hope like hell you've done your work properly."
In her report, Ms Anderson said the cost overrun was due to recruiting additional medical staff and higher-than-expected overtime costs.
"Part of the reason is in the junior doctor area where permanent staff (junior and senior) pick up additional shifts to cover vacant positions.
"The overtime charge also reflects call-backs for when junior doctors are working on call.
"There are additional costs also being incurred in the senior area where additional sessions are being worked."
In contrast, nursing costs were $489,000 better than budget for the month, partly due to staff vacancies.
Year-to-date nursing was $1,724,000 better than budget.
Other factors affecting the January result were lower-than-expected revenue, higher-than-expected outsourced clinical services spending, higher pharmaceutical costs, blood product costs and implant and prosthesis costs.
