You Do Need power plug adapter while traveling from Tahiti to Fiji.

Here is Why You Need An Adapter

The outlet and plug type used in Fiji is I and in Tahiti are C and E. This is how they look like.

Tahiti

In Tahiti, outlets of type C and E are in use.

Type C

Type E

Fiji

In Fiji, outlets of type I are in use.

Type I