However, Doc technical adviser Thomas Emmitt said the department did not have enough evidence to test claims the large bird of prey, also known as kāhu, were killed on roads in greater numbers over winter when road kill was more abundant than live prey.
Work was under way to better understand the birds’ feeding behaviour, Mr Emmitt said.
"Anecdotally, a lot of kāhu are seen scavenging on roads, particularly in winter when live prey is scarcer", he said.
"Kāhu are often killed by vehicles, which fail to slow down in time for the bird to alight.
"There is evidence of localised increases in harrier deaths where rabbit numbers are described as ‘problematic’, for example, Central Otago, Pukaki or Fairlie.
However, there appeared to be no significant change in harriers killed on roads since the 1960s, despite the massive increase in licensed vehicles on the roads, he said.
The harrier hunted live prey including small birds, mammals and insects, but also fed on carrion such as possums, rabbits and hedgehogs killed on the road.
What remained unknown was how much of their diet was made up of one or the other", he said.
A study in the Mackenzie Basin in the 1980s found the relative importance of carrion and live prey to the birds’ diet was "difficult to determine".
Doc also did not have sufficient data on the birds feeding on road kill to say whether the pattern changed seasonally, such as the claimed increase in winter.
Nor did it have sufficient data to make the call on how the seasons affected harrier road deaths, he said.
Recently, though, as part of a project looking into the impact of harrier predation on native species, Doc attached GPS trackers to several individual birds in Waikato, which might provide some insights into feeding patterns and seasonal changes.
Mr Emmitt said the species was classed as "not threatened" in the New Zealand threat classification system.
They were abundant throughout most of New Zealand including coastal areas, estuaries, wetlands, pine forest, farmland and high-country areas.
They had significant habitat overlap with many endangered native species and preyed upon adults, chicks and eggs, he said.
