Green’s Dictionary of Slang

(N.Z./UK) a bombardment.

1943		Yorks. Post 15 Dec. 6/5: An Effective ‘Stonk’ [...] just then a gunner’s ‘stonk’ - that is, all the guns on one spot — went crash into the middle of the Jerries.
1944		Halifax Eve. Courier 3 June n.p.: A mortar ‘stonk’ — over a hundred bombs concentrated in one area.
1968		G. Slatter Pagan Game (1969) 175: He never died in any stonk but met his death by drinking plonk.