The Makerua Swamp or Opiki Plains in the Opiki District of the Manawatu-Wanganui Region of New Zealand were around 1910 the location of the flax mills that once operated on the banks of the Manawatu River. Flax fibre, often known as New Zealand hemp, New Zealand flax or harekeke, but more correctly known as phormium tenax, was an important produce of New Zealand.