"They run short-staffed everyday and they choose not to get people in to cover shifts," he said.
Lives were being put in danger, Mr Hanlon said.
There were eight serious assaults on staff over the last six years at Ngawha Prison, the second highest number in the country after Rimutaka Prison with 12.
A serious assault was defined as physical violence that involved sexual assault or where the victim requires overnight hospitalisation or ongoing medical intervention which results in time off work.
Northland Region Corrections Facility manager David Pattinson said he "strongly refuted" allegations of understaffing. The prison was staffed at just over 100 per cent, he said.
"When there are unexpected staff absences, for example if someone is away sick, the operations team works with the unit to ensure it is adequately staffed," Mr Pattinson said.
"When comparing assault numbers in different prisons it is important to note that each prison has a different make-up."
For example, Ngawha Prison held 548 minimum to high security prisoners, while other prisons held low security prisoners.
"Over the last 10 years prisoner on staff assaults at [Ngawha Prison] have been decreasing and are currently at the lowest point yet," Mr Pattinson said.
There were 34 non-serious assaults over the last four years at Ngawha Prison, the third highest after Rimutaka Prison with 55 and Auckland Prison with 41.
A non-serious assault was an act of physical violence that resulted in physical injuries that may or may not have required medical attention, but not overnight hospitalisation or ongoing medical treatment.
Ngawha Prison was also second highest for no-injury prisoner on staff assaults, including spitting, throwing water at staff or pushing, with 109 incidents over the last four years. The highest was Auckland Prison with 120.
Te Tai Tokerau MP and Labour spokesperson for police and corrections Kelvin Davis said he visited Ngawha Prison yesterday and it appeared to be well run.
"I'm quite surprised there are such a high level of assaults," Mr Davis said.