i have lived in fiji for many years. on vanuwa levu (the second biggest one) and i can promise you that you'll be very lucky to find anything climbable rock there. there are the occasional limestone cliffs but they aren't in any condition to be climbed.
viti levu, where nadi and suva are, may have some climbing... i always see the cliffs on the mountains around nadi - like suggested - but i wouldn't put that much confidence in it. the faces there look about 10 meters max and every cliff looks like a mission.
rather kick back and enjoy a bowl of kava on the beach.
vinaka vaka levu, n ni sa bula.