He said it was worse because one of their members had died at the park last season.
"This type of theft brings it all home to the family and the club in general, it's not a good thing."
Stephen said they were at a "complete loss" as to why someone would stoop so low.
"It's not something you could use in any other situation and quite frankly we would like it back as soon as possible."
Less than a third of Wellington sports ground have a defibrillator on hand. A Keep Wellington Beating campaign, launched by the Liverpool supporters club, were trying to change that.
They had currently raised just over $8500 of their $25,000 goal.
Stephen said having an AED on hand gave piece of mind to everyone.
"When you have an accident on a sports field, the surety you have with a defibrillator in place can't be beaten."
The club were hoping for the safe return of the device and that anyone who knew anything would do the right thing.
"The best thing they could is encourage whoever's taken it to bring it back, drop it outside the clubrooms or drop it off to police," Stephen said.