Neafu is a provisioning desert.
No grocery stores worth the name and the markets... I gotta disagree with the other posters.... they are expensive and crap... unless you eat the palms the locals eat.
When I was there, admittedly a few years ago in 2008, they couldnt even grow an egg on the island. Eggs came in from somewhere else and you never knew when they would be available or their quality.
Tomatoes in the market are neat;y arranged in little piles of small tomatos about 3 or 5 in each pile so really, you totaly provisioning is a few piles, cocnuts, and if you are lucky a dozen eggs.
make sure your main provisions from larger countries last your trip through Tonga