The Garenne Stand has suffered from deliberate water damage which will cost thousands of pounds to put right.
Guernsey Police said three toilet sinks were blocked and the taps left on for about half an hour on Tuesday evening.
Water flooded through the floor and walls into offices below, putting them out of action because of fears about electrical connections.
The damage is not expected to cause any problems for events due to be held at Footes Lane.
Police said they were aware that the area was being used by members of the public and would like to speak to anyone who might have information that would assist in the identification of the offender or offenders.