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Sixty-four breeding pairs were found when the security site was surveyed this month and last.
Senior ranger Andrew Smart said this means the northern Fiordland site is the first in the South Island to exceed the target of 50 breeding pairs.
"This year’s number is only a minimum and there could be up to another nine pairs."
He said Fiordland’s whio are in the midst of a population comeback, thanks to an ongoing programme of predator control.
"We are only doing work over a relatively small area and this is where we are making a difference. Predator control has to be kept up and expanded for whio to have a chance to increase in number and spread over their natural range."
REPORT: LUISA GIRAO/ PHOTO:SUPPLIED