The volcano which dominates the island erupted two weeks ago but the island's residents are yet to receive any aid.
World Vision says at least a thousand Manam Islanders who were living close to the volcano have been evacuated to Bogia on the mainland but several thousand people remain on the island.
The Madang area programme manager Reuben Lulug says his team on the ground has reported back to the head office in Port Moresby but help is yet to arrive.
"There is an urgent need for the relief supplies particularly most of the water the wells have been contaminated and I think the children, most of the children are already having the effect of diarrheoa because they are drinking from those contaminated water sources."
Mr Lulug says his people on the ground say the volcano is still belching thick clouds of smoke and locals fear it will erupt again.