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

Here is Why You Need An Adapter

The outlet and plug type used in Tonga is I and in Falkland Islands is G. This is how they look like.

Falkland Islands

In Falkland Islands, outlets of type G are in use.

Type G

Tonga

In Tonga, outlets of type I are in use.

Type I