Report them to CAA - Not supposed to go below 500ft around the public except for landing etc.

Low Flying

The minimum acceptable heights for flying are prescribed in Civil Aviation Rule 91.311 Minimum heights for VFR flights. Briefly stated, the minimum height an aircraft may operate over a city, town or settlement, is 1000 feet above the highest obstacle, except when taking off or landing. Generally, this is the height at which aircraft are flown when operated within the circuit of an aerodrome.

The minimum height over any other area is 500 feet. There are a number of exceptions such as when carrying out agricultural aircraft operations, when operating within a low flying training area, and when the bona fide purpose of the flight requires the aircraft to be flown at a lower height.

http://www.caa.govt.nz/passengers/aviation_concerns.htm