Drones have become not only readily accessible but also affordable, compact and easy to operate. $2,000 can buy a drone capable of flying more than six kilometres with automated guidance, stabilisation and collision-avoidance systems and high-definition cameras.

As farmers consider how to use drones beneficially in their businesses, others on the outside are using them to peer in.

In 2013, animal welfare group Animal Liberation was reported by the ABC to have purchased a drone and used it to capture footage of the treatment of birds on a free-range egg farm. This year, the Wilderness Society used a crowd-funding campaign to raise $33,266 to buy three drones to film land clearing in Queensland, New South Wales and Western Australia. The goal: to "edit footage into compelling packages" to influence public debate and law making.

But has the law kept up to protect businesses and individuals from drone surveillance that invades their privacy?

Who regulates the use of drones?

Australian regulations, policed by the Civil Aviation Safety Authority (CASA), are concerned only with protecting the safety of other aircraft, people and property from physical harm. CASA sees the privacy implications of drone use as beyond its remit.

In the context of activist groups spying on farms, the Commonwealth Privacy Act is unhelpful, with numerous exceptions to its privacy principles for almost anything done by small businesses, political or media organisations and individual citizens.

Internationally, the United States is leading the way developing laws to protect against drove surveillance. Twelve States now regulate to address the surveillance capability of drones. So-called "ag-gag" laws limit journalists and activist groups from using drones to capture unauthorised images of farming enterprises. The Idaho version of this law, for instance, requires a landowner's permission before a "farm, dairy ranch or other agricultural industry" can be monitored by a drone.

Developments in the Australian courts

Australian courts have considered several scenarios where an invasion of privacy could invoke other actionable legal rights.

Airspace above private land may be considered private property, to the extent that the airspace is necessary for using the land below it. The entry of a drone into that space could therefore give rise to an action in trespass. However, by flying higher, the higher-resolution drone operators could at least in theory overcome any challenge.

In a leading case, the Court found that no trespass was committed by a Cessna aircraft flying over a property to take an aerial photograph, but suggested that constant or ongoing surveillance could amount to actionable nuisance.

In a case from Tasmania that went to the High Court, two unknown trespassers filmed the inside of an abattoir where possums were legally slaughtered. The recording fell into the hands of the ABC, which intended to broadcast excerpts. The abattoir owner took court action to prevent the broadcast but failed, the High Court not recognising any legal right based on an invasion of privacy but indicating that such right could develop in the future.

In particularly compelling circumstances, it could be open for a court to restrain a proposed use of drones or award damages to a landholder based on an invasion of privacy, particularly where the landholder's activities are lawful and the intrusion would be highly offensive.

Self-help?

In 2013, the town of Deer Trail, Colorado was reported in the Sydney Morning Herald to be considering licensing drone hunters and paying US$100 bounties to those who could produce the fuselage and tail of a downed drone. For whatever reason, the residents voted against the proposed ordinance.

In Australia, no law permits such destruction of another's property. A drone owner could successfully sue anyone who deliberately destroys a drone. Further, wilful damage to another's property constitutes a crime punishable, in Queensland, by up to five years' imprisonment.

As drone technology and its popularity advances rapidly, the need for laws to protect the privacy of individuals and businesses becomes even more apparent. Until lawmakers provide some certainly, the extent to which landholders can insist on privacy from roving drones remains open to debate.

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