Hi Graeme,
You had it right. The excelsa is a stowaway. It happens often. I was looking at some massive cut timber slabs used as steps the other day, on which the cracks were all lined with tiny, red-tinged seedlings of pohutukawa. They appear in all kinds of unlikely places if allowed to, and there would have been plenty of pohutukawa seeds floating around Oratia when the plant was propagated.