Green’s Dictionary of Slang

(Aus.) acceptable, i.e. not too bad.

1904		Sydney Sportsman (Surry Hills, NSW) 16 Mar. 2/3: Mr. E. A. Warburton is n.t.b. as the groggy Nero.
1904		Sydney Sportsman (Surry Hills, NSW) 16 Mar. 2/3: That giddy young thing Johnny Johnston [...] an undoubtedly clever turn and n.t.b. as a laugh-maker.