You Do Need power plug adapter while traveling from Pitcairn Islands to Sri Lanka.

Here is Why You Need An Adapter

The outlet and plug type used in Sri Lanka is G and in Pitcairn Islands is I. This is how they look like.

Pitcairn Islands

In Pitcairn Islands, outlets of type I are in use.

Type I

Sri Lanka

In Sri Lanka, outlets of type G are in use.

Type G