Keeping dogs fully fenced in may well be a council bylaw, and not be country wide. All I could find on the subject was an application for a two or more dog license, which clearly states that the property must be fully fenced to keep the dog on the property at all times, or if not fully fenced, a suitable section of ground fenced in to keep the dog in. It even says if the property is fully fenced, a suitable dog free area must be provided for official visitors!
This was Christchurch City Council, here's the form:
http://www.ccc.govt.nz/Animals/forms...enceAppFrm.pdf