A major security breach where shotguns were accidentally delivered to inside the Northfield Women's Prison has sparked an investigation by South Australia's Corrections Department.

9NEWS can reveal the shotguns were in an unmarked box inside a delivery truck, and made it through security screenings undetected.

It wasn't until prison guards were unloading the delivery, in an area frequented by inmates, that they were discovered.

The prison was sent into lockdown and the guns removed without incident.

Neville Kitchen from the Public Service Association told 9NEWS the mistake should never had happened and it was seriously concerning for his members.

It's understood a mix-up with the delivery address contributed to the error.