She said the Koha Shed would have to close for a day while the rubbish was cleared and dumped because it was already short staffed this week.
The charity is in the process of installing security lights and cameras that will capture images of intruders and send them to staff members' phones.
"Be aware that anyone seen dumping can expect to be tracked down and their rubbish returned.
"They will also receive a trespass notice."
The Koha Shed hopes to reopen tomorrow, October 31, and Sunnex said she wished to apologise to people who could not visit earlier.
"It's the few who spoil it for the rest," she said.
The Koha Shed is usually open at 88 Duncan St, Whanganui East from 10am to 3pm on Tuesdays, Wednesdays and Thursdays.