A report warns that infrastructure and services are not keeping up with urban growth in many areas.
UNICEF'S Dr Isiye Ndombi has told Radio Australia's Pacific Beat poverty no longer means a child in a rural village, but often a child living in slums, settlements or shantytowns where infrastructure can't keep up with growth.
"In the Pacific now we are talking about a quarter of the population living in urban or urbanising centres, and at least half of the Pacific Island countries are can we say to be already urbanised," he said.
"So that's the big challenge."
Dr Ndombi says low wages and the high cost of living in the Pacific means many families can't afford basic necessities.
"You have to develop the rural areas so that you minimise the urban drift," he said.
"The attraction of the urban areas can be minimised if the rural areas have some of the same services that push people towards urban areas.
"But the second thing is that the urban areas need to be better planned. When people move into urban areas and settle where services are not good, so young people are growing up in unplanned urban settlements, that take a lot away from them."
The report has found many young people are growing up in settlements which do not have sufficient water, sanitation or food
Dr Ndombi says the new circumstances also mean they don't have access to traditional resources.
"What we used to call the subsistent affluence in the Pacific, where the people could fish, they could pick up coconuts, they could go to the taro farm and get the food, when they are in an unplanned urban area, they don't have that recourse," he said.
"So the risk of malnutrition is higher...there is inadequate shelter...and the risk of child labour and trafficking is higher."