The common law has constrained the ability of police officers to search suspects, the greatest power being in relation to suspects who have been arrested. Arrested people may be searched if there is a possibility that a weapon will be found, in the interests of preventing harm. There has been no general common law power to search people, and where powers of search exist they have been created by statute. For example, s 202B of the Crimes Act 1961 [NZ] allows a police officer to search a person if there are reasonable grounds to believe that an offensive weapon will be found.
The Supreme Court of Canada in Mann v R
 SCC 52 (23 July 2004) has changed the common law by permitting a police officer to "pat down" a person if the officer can (if challenged to do so in court) articulate a reason why such a procedure was necessary to protect the safety of the officer. Such "pat down" searches occur without necessarily involving the detention of the person, and consequently rights that come into play upon detention do not arise. The officer must, of course, be acting in execution of duty in speaking to the person, and this includes the general duty to prevent and investigate crime. The person need not therefore be suspected of committing an offence.
Plainly, while it is sensible to allow officers to ensure their own safety while they act within the law, difficult issues of fact may arise because of the fine line between patting down and ascertaining the contents of a pocket. Such a line was breached in Mann
, where the officer reached into a pocket where he had detected something soft (which turned out to be a bag of cannabis); he had gone beyond acting to secure his safety. There could easily be disputes over whether the person consented to revealing what was in his pocket, although it must be acknowledged that the courts are used to determining such disputes. It is, of course, inherently unlikely that a person would give free consent to a search, if he knew that the officer had no right to require it, when it would inevitably incriminate him.