1. Originally and chiefly English regional (northern). Something large or substantial of its kind.
1862 - ‘A plonker’ is an article having extraordinary substance. A piece of woven material unusually thick is ‘a plonker’. C. C. Robinson, Dialect of Leeds & Neighbourhood 386
1903 - That turnip's a plonker English Dialect Dictionary vol. IV. 550/1
1950 - I lost another much bigger one , a real plonker! D. Eastwood, Diary 6 March in River Diary 24
2. Australian Army slang. An artillery shell.
1917 - We did fatigue work at the front—night work at first,..when Hoch used to give us a plonker or two every now and again. Richmond (New South Wales) River Herald 25 December 2/6
1920 - I had helped him out when he had got chucked into a shell-hole by a plonker. Aussie: Cheerful Magazine December 36/3