Brunei Darussalam became independent in 1984, and first participated at the Commonwealth Games in 1990, held in Auckland, New Zealand.
There may have been a participant from Brunei at the 1958 British Empire and Commonwealth Games in Cardiff, but Brunei is not shown on the medal standings for that Games[4], however the programme for the Opening Ceremony of the 1958 games shows Brunei in the list of teams participating.
References
^Opening Ceremony programme of the 1958 Commonwealth Games lists Brunei as competing