You Do Need power plug adapter while traveling from Hong Kong to Pitcairn Islands.

Here is Why You Need An Adapter

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

Hong Kong

In Hong Kong, outlets of type G are in use.

Type G

Pitcairn Islands

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

Type I