Pacific Islands Climate Action Network convenor Shirley Laban says the decision not to attend the meeting is alarming and disheartening.
The summit is intended to provide momentum for a global pact to tackle climate change, with an agreement expected to be finalised in Paris next year.
More than 120 world leaders, including US president Barack Obama and Britain's prime minister David Cameron, are expected at the meeting.
Ms Laban says climate change presents a clear and present danger for Pacific countries.
She says the absence of the two prime ministers sends the wrong message.
"When we have our neighbours like Prime Minister Tony Abbott or John Key not attending it gives us a great disappointment. What will that mean? Do they have the concern for us as neighbours or what?"
PICAN's Shirley Laban.
