We have two bases for arrests as security guards. One is the citizen's arrest, just as any other citizen in the age of majority can do. The other is as the owner of private property dealing with, say, a trespass issue in an apartment building, or even in a mall, where we have the delegated rights of the owners or occupiers or managers to tell people to leave the premises if, for instance, within the retail environment, they set up a stall and they're selling products in the mall without renting from the mall.
If we tell someone to leave and they don't leave when directed to do so—this is a direct quotation from the legislation—then we can arrest without warrant, and we can arrest for indictable offences, as citizens.
That's where it comes from.