A Department of Conservation programme to boost numbers of the world's rarest kiwi, the Okarito rowi, has been so successful they are now running out of room in the forest.

When the egg recovery project began in 2006, it was estimated that fewer than 200 rowi existed, all of them in the Okarito forest.

DoC aims to increase the population to 600 by 2018, helped by removing eggs off the nest, hatching them at Franz Josef Glacier, and sending the chicks to a predator-free island until they are big enough to be released back at Okarito and fend off stoats.

Last October, a record 50 birds were returned to the forest.

DoC South Westland senior ranger biodiversity Lyndon Slater said that as well as hatching eggs in captivity, trapping programmes and poison operations had contributed to the increase in numbers of rowi in South Okarito Forest.

"South Okarito is assumed to be at or near its carrying capacity for rowi, based on adult territory size and current population estimates," Mr Slater said.

North Okarito Forest still had room for more kiwi; it had a higher proportion of young birds, many of which would not have established territories yet. Over the next few years, DOC would look at where it could establish populations of rowi at new sites throughout the species' former range in South Westland.

"It's not a problem at the moment, but as the population has been growing and will continue to do so, we want to make sure that rowi have space to thrive in the future," Mr Slater said.

Rowi could have very large territories, up to 100ha each, though more often they would roam over significantly smaller areas.

Aggressive behaviours and poor breeding results could occur if too many adult birds, which were all competing to establish territories, were squeezed into an area, he said.

- Greymouth Star