Botanical name: Rumex obtusifolius
Family name: Polygonaceae
It is a perennial weed which has a large tap-root system. It can vary in size quite considerably, sometimes surviving in regularly mown lawns by producing small, prostrate leaves, but usually growing quite large in pastures and waste areas, out-competing surrounding plants by covering them with its broad leaves. (Massey Uni Weeds Database).
Young plants found growing under the fence in backyard.