[2] The largest tuff ring in the field is Onewhero maar which is 2.7 km (1.7 mi) in diameter and was formed 880,000 years ago.

[3] The field can be divided into three broad geographic areas: the north-eastern section, which consists of eroded remnants of lava flows and scoria cones, some of which are found in the south-western Hunua Ranges, the southern section to the south of the Waikato River, which features many of the more recent and better preserved scoria cones, and the central section, which forms much of the low-lying land of the Pukekohe area, between the Manukau Harbour and Waikato River.

To the south and east, visible on the horizon from the Bombay Hills are back arc volcanoes.

These include the volcanoes of the Taupō Volcanic Zone to the south which have now been continuously active for over 2 million years.

The unnamed most northern of these was presumably followed by the basaltic extrusion that extended all the way to the Morley Road tuff rings.