To be 'done' for Trespassing in NSW by the owner they would have to sue you in a Civil Court. The owner of the land can then be awarded costs for any damage or loss he/she may have incurred as a result, and/or compensation in vindication of his right to exclude you regardless of damage or loss. I do not know if there has to be some sort of obvious boundary or not. I guess that would be up to the court at the time. One would think that if there was no obvious or reasonably recognised boundary between the common land and the private land that it would be difficult to prove a matter of trespass against you. Remember that Civil Law is based on the balance of probabilities and not beyond reasonable doubt.
To be charged criminally by the police there is a bit of a difference. The charge is 'Enter enclosed lands' under the Enclosed Lands Protection Act 1901. This requires there to be some form of restriction on the boundary of the property and is defined as follows:
"any lands, either public or private, inclosed or surrounded with any fence, wall or other erection, or partly by a fence, wall or other erection and partly by a canal or by some natural feature such as a river or cliff by which its boundaries may be known or recognised, including the whole or part of any building or structure and any land occupied or used in connection with the whole or part of any building or structure." (Lawlink, NSW)
Section 4 of the Act contains the offence provision and provides as follows:
(1) Any person who, without lawful excuse, enters into the inclosed lands of any other person, without the consent of the owner or occupier thereof, or the person apparently in charge of the same or remains upon the inclosed lands of another person after being requested by the owner or occupier or person apparently in charge of those lands to leave those lands, shall be liable to a penalty not exceeding $100, and the proof of such lawful excuse shall be upon the defendant in any such case.
A case in point here is that if you broke into a house because you wanted somewhere to sleep, didn't damage anything in doing so and didn't steal anything or intend to steal anything you would only be charged with Entering Enclosed Lands.
This is not legal advice and I am not a solicitor, nor do I advocate purposely trespassing. This is just how I understand these laws and thought you may be interested in the event you were caught accidentally being in the wrong location. A little long I know, sorry.