Most councils have bylaws regarding the depth of the pond being built. 18" is the usual maximum (which is why the fibreglass and plastic ones you buy are so shallow) before any consents need to be sought but this varies from council to council. I have 2 baths sunk into my garden which, theoretically, should be fenced!
In Marlborough, only 18" is too shallow as it gets too hot here unless you build in a shaded area.