Can an agent with power of attorney override the principal?

This is a matter for the police in your area, not for a Forum.

This post doesn't make much sense, really. Your father is the principal, you're saying, but gave power of attorney to his niece, who calls the police on you when you go on dad's land, with his permission?? I'd call an Elder Care Attorney for advice moving forward if I were you, and get some real legal guidance rather than asking an internet forum for advice w/o providing all the details. Something seems amiss here to me. You claiming your father 'gives you permission' to be on this land is here-say, really, vs. his niece who has legal POA is saying you do not; her word, therefore, is NOT here-say, and prevails, I should think.