Overnight on Thursday, six multi-coloured plastic squares in circular timber frames in the Elderslie summer house - which is next to the display house near the Chelmer St entrance to the gardens - were smashed.
At least two concrete plant pots were overturned and dumped inside the summer house, and other plants, including rosemary and annual flowers, were ripped out of their pots.
Sergeant Blair Wilkinson, of Oamaru, said the damage was discovered by council contractors early yesterday.
Work to repair the Heritage New Zealand category 2 building and clean up the damage started soon after.
A council contractor, who did not want to be named, was dismayed by the damage and said those responsible had showed complete disregard for the hard work that went into maintaining the popular attraction.
"It's going to cost the community.
"It's all extra work that has to be done, but the vandals don't give a crap about that.
"It is frustrating ... it doesn't happen often, but it is frustrating for the staff because they work very hard to make it look good."
The contractor said it was "fortunate" the timber trellis was not damaged, which would have added to the already "delicate and expensive work".
Coloured plastic, which replaced what would have originally been stained glass, would have to be sought, which the contactor said was not as easy as it sounded.
Waitaki District Council parks officer Jane Matchett said council records showed the summer house was built in 1877 and given to the gardens in 1947 by the then owners of Elderslie, a sprawling estate near Windsor in North Otago.
