Green’s Dictionary of Slang

tab, the n.

[abbr.]

(Aus./N.Z.) the Totalizator Agency Board.

[Aus]N. Keesing Lily on the Dustbin 61: Both [sexes] patronise the TAB (Totalizator Agency Board) often known as ‘th’ tab’.
[NZ] McGill Reed Dict. of N.Z. Sl.