Hi Nick,
it will depend on the origin of the canvas, if it's cheap indian or chinese canvas, the best advice is to take it off and replace with Quality Australian made canvas.
Assuming it is Australian made canvas they are all proofed, so you don't need to do anything from a proofing point of view, the best quality proofing is Dynaproof - a Wax Convertors technology.
But you should put the canopy on and secure tightly, then wet (and I mean really wet) the canvas and dry it 2 to 3 times to set the canvas properly - ie tighten up the weave and get the thread to fill up the stitching holes.
then you'll have a water proof canvas that fits correctly and is resistant to further shrinkage
cheers
Scott