The trail of damage included a smashed television, a pool of urine on the floor and food smeared into the carpet.
St John believes the break-in happened sometime overnight on Sunday. It has been reported to police.
Regional GM St John South Island Craig Stockdale said it was disappointing that the charity's public space was vandalised.
The damage has forced a meeting of the charitable organisation's youth division to be postponed until next week.
A public first aid course that was also due to be held in the facilities on Monday had to be relocated.
Stockdale said St John was appealing to the public to report any suspicious behaviour to police.