You Do Need power plug adapter while traveling from Cambodia to Cocos (Keeling) Islands.

Here is Why You Need An Adapter

The outlet and plug type used in Cocos (Keeling) Islands is I and in Cambodia are A, C and G. This is how they look like.

Cambodia

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

Type A

Type C

Type G

Cocos (Keeling) Islands

In Cocos (Keeling) Islands, outlets of type I are in use.

Type I