But Brown said the hospital did not fully lose power, thanks to a third line of defence.
"Firstly, it's unacceptable the backup generator failed, but thankfully, our hospitals have battery backups on top of that, to ensure that there is the ability to maintain life preserving services," he told reporters at Parliament, just moments ago.
He said he had formally requested an investigation into what went wrong at Nelson Hospital.
"I have asked the Health New Zealand board to undertake an independent investigation into what happened here, and also to audit all the backup generation across all of our hospitals," he said.
Brown also thanked the unionised health workers of Nelson who went into the hospital on Thursday, instead of striking. Members returned after hearing power was out so they could step in if needed.
"Look, I'm very grateful for all of the work that was undertaken. A number of union members who were striking came back to the hospital to provide additional support.
"I'm very grateful for the effort that they provided in these circumstances. Is it acceptable? No, that's why we've asked for an independent investigation to what happened here," he said.