The New Zealand Prime Minister agrees in part with an assessment by American diplomats that Solomon Islands would lapse into chaos without the Regional Assistance Mission.
Leaked American diplomatic cables say nearly a billion US dollars and eight years of effort by Australia have failed to bring stability.
New Zealand is one of the RAMSI partners, providing money and personnel.
The Sydney Morning Herald says the information was contained in assessments from the US Embassy in Port Moresby leaked to the whistleblower website Wikileaks.
It quotes the cables as saying Solomon Islands would relapse into turmoil within weeks, if RAMSI withdraws.
Mr Key says it's complex, but agress the Solomons is not yet stable.
He says New Zealand believes RAMSI should continue.