Most trespass is under the following
Inclosed Lands Protection Act
4 Unlawful entry on inclosed lands
(1) Any person who, without lawful excuse (proof of which lies on the person), enters into inclosed lands without the consent of the owner, occupier or person apparently in charge of those lands, or who remains on those lands after being requested by the owner, occupier or person apparently in charge of those lands to leave those lands, is liable to a penalty not exceeding—
(a) 10 penalty units in the case of prescribed premises, or
(b) 5 penalty units in any other case.
There is an implied right of entry for a lot of scenarios and absent that implied right then one needs to refuse to leave after being asked to leave. Then you can call the Old Bill or maybe consider s 100 of LEPR Act
100 Power of other persons to arrest without warrant (cf Crimes Act 1900, s 352)
(1) A person (other than a police officer) may, without a warrant, arrest a person if—
(a) the person is in the act of committing an offence under any Act or statutory instrument, or
(b) the person has just committed any such offence, or
(c) the person has committed a serious indictable offence for which the person has not been
tried.
(2) A person who arrests another person under this section must, as soon as is reasonably
practicable, take the person, and any property found on the person, before an authorised officer to be dealt with according to law.