If you look in the
Yellow Pages for ply suppliers in Brookvale, you'll find there are almost a dozen shops around the area. I was in the Bruynzeel shop back in Feb/Mar and they had a huge range of okume/gaboon marine plywood, even small sections of various thicknesses which was ideal for the paddles I was making. So one the one hand, I would be surprised you couldn't get the stuff you're after.
On the other hand, I don't think you particularly need the specific wood mentioned in the plans. I suspect its more ideal to go with okume because of the weight thing. But for a small PDR, this might not be an issue for you. And once you've coated everything in epoxy, you'll have the watertight factor which negates a 'marine ply' label.
Really, this is purely opinion, so I look to be corrected by the professional builders if necessary.
Another opinion is that Hardware and General in Brookvale is one of the best hardware store in Sydney. Go where the real tradies go! (I'll take any discounts due to this blatant plug too Mr MacGregor!)