Swabs taken from two cells after the incident in late May were sent to Environmental Science and Research for analysis but facility manager Jack Harrison yesterday said screening tests had detected no toxic substances.
Further tests carried out last week week to try to determine what happened had also returned negative results.
"This, unfortunately, leaves the cause of the incident a mystery."
The corrections officers had been conducting routine searches in two adjacent cells.
Soon afterwards, they began complaining of sore throats, nausea, headaches and blurred vision.
They were sent to Dunedin Hospital for treatment and were given time off work to recover.
Meanwhile, 200 people from the lower South Island have been approved as visitors to the facility open day this month.
The temporary guests of the 335-bed facility will be coming from Oamaru, Dunedin, Invercargill "and everywhere in between", Mr Harrison said.
Members of the public have until Tuesday to apply for a spot.
The open day will be held on Saturday, June 27.
