Vanuatu's opposition leader, Edward Natapei, says he has grave concerns at the decision by the Melanesian Spearhead Group giving Indonesia observer status.
The MSG is meeting in Suva with a leaders' summit set for next Thursday.
Mr Natapei, who is the previous MSG chair, says making Indonesia an observer is a counterproductive move.
He says, instead, the Melanesian people of the disputed Indonesian region of Papua, who have long pushed for observer status, should be given it and eventually made full members, in the same way as the Kanaks of New Caledonia are represented.
"I would express grave concerns about this initial move to bring Indonesia in as on observer of the MSG, because that is not going to help the cause of the Melanesian people of Indonesia, the West Papuans."
Mr Natapei says supporting New Caledonia's Kanaks in their push for independence was a key reason for the establishment of the MSG nearly 30 years ago.