You Do Need power plug adapter while traveling from Thailand 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 Thailand are A, B, C and O. This is how they look like.

Thailand

In Thailand, outlets of type A, B, C and O are in use.

Type A

Type B

Type C

Type O

Cocos (Keeling) Islands

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

Type I