The money is to be spent on education, health and infrastructure, such as rolling projects in renewable energy providing solar power to Cook Islanders' homes.
On top of the three-year aid pledge Key also announced a gift of $11.7m (US47.6 million) to go towards rebuilding the country's national secondary school.
Key has led a delegation of ministers and MPs and New Zealand-based Cook Islands community leaders to Rarotonga where they were welcomed by Prime Minister Henry Puna for two days of meetings and festivities.
The $11.7m will help rebuild Tereora College - the Cook Islands' biggest priority in terms of school infrastructure, which holds 600 students.
The cash injection will help fund the initial design of the college's redevelopment and will cover construction of an administration centre, library and technology block.
Key said infrastructure was a big priority in the Cook Islands and New Zealand was a big supporter of getting the country away from fossil fuels and using renewable energy.
Puna was a “great friend” of New Zealand and it was a relationship of “trust friendship and openness”, Key said.
When Puna was asked whether he thought New Zealand was providing enough aid given the mass migration of Cook Islanders to other shores, he said he couldn't stop people from leaving.
“It's a mistake to associate any help from New Zealand with the fact people are leaving the country.
“Cook Islanders are voyaging people, they travel.”
Puna said the nation did its best to try to provide a reason for people to stay, including “stimulating activities”.
Key said the two countries had been working together to try to bolster the Pacific's economy to encourage people to stay.
Part of that work was legislation passed earlier this year allowing people to apply for New Zealand Superannuation while they are resident in the Cook Islands, Niue or Tokelau.
Previously a person had to be “resident and present” in New Zealand at the time of their application.
People wanting to return home to the islands to retire will be able to do so after the age of 55 without losing their eligibility for New Zealand superannuation.
When Key was asked whether the Government would consider loosening that to the age of 50 he said it was unlikely in the near future because of financial implications.
He said the Government was already under pressure to extend the scheme to other Pacific nations that weren't currently included, which would only make it more expensive.