The swamp harrier only became established in New Zealand within the last few hundred years, after lowland forests were extensively cleared by the first Polynesian settlers.

[7] According to archeological and genetic research, humans arrived in New Zealand no earlier than about 1280, with at least the main settlement period between about 1320 and 1350,[8][9] consistent with evidence based on genealogical traditions.

All swamp harrier remains from New Zealand that have been studied in detail are some 1000 years old at most, meaning that they post-date human settlement.

[12] Common enemies of harriers are Australian magpies, masked lapwings which dive bomb and European starlings which mob individuals.

The swamp harrier mainly feeds on ground birds and waterbirds, rabbits and other small mammals, reptiles, frogs, and fish.

During the winter months harriers feed to a large extent on carrion, including roadkill, frequently falling victim to vehicles themselves.