Green’s Dictionary of Slang

acker n.2

also ack

(Aus.) acne.

[Aus]N. Keesing Lily on the Dustbin 176: ‘Zits’ or ‘ackers’ (pimples) may be ‘subterraneous’ or ‘volcanos’ and acne sufferers have faces ‘like a festered pickle bottle’.
D. Oswald Dags 1: Full-scale ack-attack — face like a relief map of Switzerland, you know.
‘Language Talkback’ at Oct. [Internet] There are many types of diminutives in Australian English [...] acker: acne (=a pimple).