As it is in most states I think, something to do with sounding an emergency warning device when not in an emergency/urgent situation
Correct oh stern ruler. The horn is considered a warning device and therefore there is an offence for using an warning device
TO(RUM-RR)Regs 2009 - Division 3 Horns and radar detectors
s224 Using horns and similar warning devices
A driver must not use, or allow to be used, a horn, or similar warning device, fitted to or in the driver’s vehicle unless—
(a) it is necessary to use the horn, or warning device, to warn other road users or animals of the approach or position of the vehicle; or
(b) the horn, or warning device, is being used as part of an anti-theft device, or an alcohol ignition interlock, fitted to the vehicle.
Maximum penalty—20 penalty units. (Penalty Unit =$110)
Link to Queensland legislation:
http://www.legislation.qld.gov.au/Acts_SLs/Acts_SL_T.htm