You Do Need power plug adapter while traveling from Mauritius to Tonga.

Here is Why You Need An Adapter

The outlet and plug type used in Tonga is I and in Mauritius are C and G. This is how they look like.

Mauritius

In Mauritius, outlets of type C and G are in use.

Type C

Type G

Tonga

In Tonga, outlets of type I are in use.

Type I