Bob. Not much help to our friends from the Republic but just to add that I believe the NT have turned funny about shots taken on their property of their property which have been destined for sale. I think AP ran an article about it a few months ago.
Like you I have never had an issue actually taking photos but have never used a tripod so was probably assumed to be an amateur doing it for my own pleasure.
Unless you had a site or gallery full of NT shots and were on the radar of an NT official who knew of your existence, I'd think the chances of the NT mounting a case would be very slim.