Following the lunch break they were observed by staff to be exhibiting "concerning behaviour, including becoming unbalanced, confused and vomiting".
The women were removed from the work site and seen by prison health staff.
The site was locked down as a precaution and six staff accompanied them to hospital.
One woman returned to prison that evening where her health was monitored, but two were admitted to hospital overnight and discharged the following day.
There were no ongoing health concerns for any of the prisoners involved, Alleyne said.
The women were interviewed and admitted eating various plants and bugs, including a plant from the datura species, a powerful hallucinogen that can be deadly.
"The plant was removed from the tunnel house and destroyed immediately," Alleyne said.
"Further checks have been completed across the grounds to ensure that there are no other similar plants on site."
Corrections had launched an operational review to confirm how the plant came to be growing on the grounds.
"The plant is a known weed and a number of years ago it was eradicated from the grounds of the prison," Alleyne said.
The women had been charged with misconduct following the incident, but a mother of one of them told the Herald they had no idea they were consuming a hallucinogen.
She also questioned how Corrections could let the plant grow there in the first place, and criticised the department for not alerting her after her daughter was hospitalised.
"They had no idea it was datura, thought it was just a flower, they didn't even know what it was until afterwards.
"They were just messing around, daring each other to do silly things."
She only heard about the incident after her daughter had recovered and called her, the mother said.
"She said she nearly died, she was vomiting and her heart nearly stopped. The prison has health and safety responsibilities, how could they let this happen? And then they didn't even let me know she was taken to hospital. What would they have said if she died?"
Datura is one of the most dangerous plants in New Zealand.
It is sometimes eaten by people wanting to experience hallucinations, caused by the plant's powerful alkaloid chemicals.
But these have other side-effects, including over-stimulating the heart and acting as powerful muscle relaxants, which can be lethal.
In New Zealand datura has been responsible for admissions to intensive care, and indirectly caused at least two deaths from drowning.
Alleyne said the women's charges will be heard by a Hearing Adjudicator.
"If the charges proceed and they are found or plead guilty they may be sanctioned with a loss of privileges such as television or hobby materials, forfeiture of earnings, or a period of cell confinement.
"Further actions may be taken in response to the findings of the review."
