A very bad and very bumpy road is the only option as far as I know. Gets torn to hell from all the Fiji Water trucks rain and whatnot, and gets little maintenance. It doesn't look super far on a map, but it will take a at least a few hours because you won't average more than 25-30mph much of the time.
I recall the one time I stayed and dove at another resort very near Voli Voli back in 2009 we hired a van with a trailer to carry us and all our luggage. The roads broke one of the trailer's wheels on the return journey and we had to wait for a replacement to arrive. We luckily left plenty early so didn't miss the flight.