The Morten-in-Marsh, Gloucestershire garden was targeted through its landline telephone voicemail answering service after closing last Friday, 7 October.
Arboretum operations director Stuart Priest said: "We have one of those phone systems where when you call it gives you four options to choose different parts of the arboretum and we think that’s how they hacked in. They spent Friday and Saturday ringing Tunisian mobiles and it cost us £550 which is appalling."
However the garden was saved from further theft by its phone call provider 2 Circles Communcations, which noticed the unusual activity and called Priest on 8 October. 2 Circles blocked all calls and later, changed that to international ones until Batsford’s physical phone system supplier Aonix could change passwords and fit extra security on Monday.
"The point is when the phone system was installed 10 or so years ago, it probably had factory settings. I think we were lucky to get away with £550, we could’ve been taken for thousands and we are a registered charity. We just want to warn people that it could happen to them.
"Everyone with an automated phone answering system beware and get your security checked. We were told that one company had a £5,000 phone bill by the Monday morning after they hacked into their phone system on a Friday evening."
However, despite the fraud, Priest is in a positive mood at Batsford as he said that the arboretum's autumn colours are particularly good this year, after a spell of the right type of weather to produce vibrant leaf colour.
"We expect it to be very busy this weekend and next weekend," he said.