Glen Riley, of Penguin Place, said the birds had put on more than 2kg and now weighed up to 5.5kg, a safe weight for them to be released in the Catlins.
A lack of feed, possibly caused by the La Nina weather pattern, meant some penguins struggled to feed their chicks, Mr Riley said.
The 4-month-old penguins have spent nearly a month at the Otago Peninsula penguin hospital, after an initial fortnight with Department of Conservation Owaka ranger Cheryl Pullar.
They were fed three times a day when they first arrived, reduced to twice a day as their digestive systems improved. They had been handled as little as possible to protect their feathers.
It had been a busy summer for the hospital, with up to 50 injured or underweight birds staying at a time. Penguins were not social birds, which caused a few tensions between them.
