Ross Knoll (73 n.o.) and Matt Johnston (59) took apart the Centurions bowling line-up on Saturday, as the Dragons posted 342 from 90 overs.
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Clint Hinchliffe was the pick of the bowlers with four wickets, while Josh Anderson took three, as the Centurions battle to keep their grip on a top-four spot.
Joondalup is third on the ladder with one round to play but Perth, well placed to beat University, is just three points behind.
Anderson took three quick wickets to halt Willetton who had started well to reach 0-90, before slumping to 4-111, with Simon Branston removing the dangerous D’Arcy Short for a duck.
Willetton were 6-197 before Johnston and Knoll’s runs lifted the visitors beyond 300. Joondalup struggled without James Boyland.