Specifically methylates the adenine in position 1618 of 23S rRNA. adenosine(1618) in 23S rRNA + S-adenosyl-L-methionine = H(+) + N(6)-methyladenosine(1618) in 23S rRNA + S-adenosyl-L-homocysteine Belongs to the methyltransferase superfamily. METTL16/RlmF family.