Access the Ellerslie Panmure Highway and follow it until you are to the south of Mount Wellington (ie the cone is to your left). Turn onto Mountain Road and follow this along and round a ninety degree right hand bend. Turn left up Maungarei Memorial Drive which circles up the steep side of the cone to the car park.
If the ground is not too muddy, it is possible to take a wheelchair across the flat field on top of the reservoir, to get a view into the crater.