Origin of the Wairere Boulders from the Geology Department of Auckland University:

The boulders in the Wairere Valley are the erosional remnants of a basalt flow that has been dated as approximately 2.8 million years old. We have two dates for these so - called Horeke basalts : one at 2.84 and the other at 2.67 million years. There is a slight difference in chemistry in the two rocks that have been dated suggesting that the eruption took the form of several flows that once blanketed the high ground to the east of Horeke. The hard basalt blocks on softer sediments accumulate in valleys and look as if they are - as you describe it flowing down towards the sea.

The unusual fluting that makes the basalt surfaces look in places like eroded (karst) limestone is the result of chemical leaching by the very acid soils generated by the Kauri forests that used to exist in the area. Such fluted basalt surfaces occur in a few other places in Northland. A very long time ago someone wrote a paper on the fluted surfaces of basalts of Northland and that is what he concluded.
Glaciation during the ice ages did not extend into the North Island - except perhaps for a possible glacial moraine that has been described in the Tararua Ranges (bottom of the North Island) and possibly also on Mt Ruapehu.

We certainly know that there have never been glaciers anywhere near Northland because there are many examples of volcanoes and lava flows in the Bay of Islands and Whangarei area that date back to 10 million years but are still in quite good condition that would have been wiped out if there had been any glaciation. Also some old erosional surfaces, which have been covered by lava flows known to be many millions of years old.

The vegetation in Northland 2 - 3 million years ago is uncertain but it is believed to have been Kauri forest, and beech forest during the ice ages; there are some remnants of the beech forest in the Hunuas just west of Auckland.