I left Fiji last year after volunteering for 2 years in a Children's Home/Orphanage. I spoke to Fiji Immigration 3 times in regards to a volunteer visa, there is no form for a volunteer visa & no one at immigration seemed to know anything about it. In the end I volunteered without one (only because I couldn't find any information on how to get one!). Getting volunteer work in Fiji is very easy. I simply rang the Home and asked if they were in need of a volunteer. This lead to an interview (I didn't have any experience working with children, this was more of a meet & greet). I had to hand over copies of my passport, visa, police check etc so that the Home could pass my details onto Human Services. Then I started 'working'. I also arranged for other expats to do odd jobs around the Home.
Unfortunately many of these organizations (Children's Home, SPCA, Shelters etc) receive very little government funding, so they rely heavily on donations. As much as they would like to hire more help, they just do not have the funds for it, so volunteering is a great help to them.
Good luck & happy volunteering!! It really is a life changing experience 