Hi Mark,
Hate to see someone go unanswered but do you realise that we (NZPCN) are about promoting native plants not killing them?
Assuming your ID is correct, Hypolepis ambigua, there must be something very unfavourable with the soil that between grazing and vigorous exotic grasses it is a problem. I have no idea what the farm is like but if it was mine, I'd be getting soil tests done and thinking about whether the area affected is ever going to be that good for pasture (steep, boggy, cold?).