Background Unmet need for family planning in the Pacific is among the highest in the world. Better understanding of required investments and associated benefits of increased access to family planning in the Pacific may assist prioritisation and funding. Methods We modelled the costs and associated health, demographic and economic impacts of reducing unmet need for family planning between 2010–2025 in Vanuatu and the Solomon Islands. Baseline data were obtained from census reports, Demographic and Health Surveys, and UN agency reports. Using a demographic modelling program we compared a scenario of “no change in unmet need” with two distinct scenarios: 1) all family planning needs met by 2020; and, 2) all needs met by 2050. Results Meeting family planning needs by 2020 would increase prevalence of modern contraception in 2025 from 36.8 to 65.5% in Vanuatu and 28.5 to 37.6% in the Solomon Islands. Between 2010–2025 the average annual number of unintended pregnancies would decline by 68% in Vanuatu and 50% in the Solomon Islands, and high-risk births would fall by more than 20%, averting 2,573 maternal and infant deaths. Total fertility rates would fall from 4.1 to 2.2 in Vanuatu and 3.5 in the Solomon Islands, contributing to slowed population growth and lower dependency ratios. The direct cost of reducing unmet need by 2020 was estimated to be $5.19 million for Vanuatu and $3.36 million for the Solomon Islands between 2010–2025. Preventing unintended pregnancies would save $112 million in health and education expenditure. Conclusions In small island developing states such as Vanuatu and the Solomon Islands, increasing investment in family planning would contribute to improved maternal and infant outcomes and substantial public sector savings.