This will include looking into relevant legislation, the agrichemicals, planning and recording as well as the use, storage, transportation and disposal of chemicals.
Whanganui Prison's assistant director, Elizabeth Manchee, said the training programme was win for all involved.
"A Growsafe certificate is a highly sought after qualification. It's the only qualification of its kind in the country and is recognised by many organisations including local and regional councils, roading contractors and the Department of Conservation."
The supervisors underwent a day-long session and had to complete three lots of spraying in their own time, recording their spraying in a diary.
The qualification means these supervisors may apply and supervise the use of chemicals in the workplace.
Whanganui/Taranaki district manager Coralea Easther said the certifications had further upskilled supervisors.
"It will be particularly useful at various tracks and walkways we manage when supervisors may need to spray weeds. The flow-on effect of the training will benefit community-based offenders who could also be trained in the future."
Corrections said their top priority was to reduce reoffending and, helping offenders gain employment could reduce this likelihood.
Prison stats:
(As at September 2016)
* 9798 total number in prison.
* 1233 classified as high security prisoners.
* Most common offence - violent convictions accounting for 3686 prisoners.
* Maori account for 4989 prisoners, followed by European at 3153.
* Most common prisoner age 20-29 years.
* Mt Eden is New Zealand's largest facility housing 1026 prisoners, Manawatu Prison currently has 283.