I can send seeds for Giant Russian Sunflower if you like. Email me directly.

A large, though not monstrous, cucumber is "Giant Russian", from Eden Seeds. Beefsteak tomatoes come in various sizes. Why not ring Eden Seeds and ask which are their biggest. Many tomatoes with 'giant' in the name are so called because of the plant rather than the fruit.
Noting where you are, fruit fly might be a problem, especially with large tomatoes. Why not make the tomato one a "biggest yield by total weight" competition and grow a cherry tomato like "Golden Grape", also from Eden Seeds. Just a thought.

Thanks for the offer Pepino, I've sent you an email.

Thanks Raymondo, cucumbers would be good as they are so easy to grow.

Most catalogues that I've seen don't seem to give a good description.

I guess I'm lucky but I never seem to have a problem with fruit fly in my tomatoes and I've grown them all year round from Brisbane to here. The main problem I have are big green caterpillars eating holes in the tomatoes but my kids usually keep them under control (typical kids - like grubs). I do have trouble keeping water up to them in the middle of our hot summers but I can't stand store bought tomatoes and am not keen on cherries so I think it is worth the struggle.

I do like your idea on the biggest yield. Might have that as one category in the competition.

Dont forget Zucchini they grow huge if you dont pick them I let them grow till they are around 60cms long.
Then sliced them in half lenghtwise for the chooks to enjoy, not good for us humans by then too stringy.
But Ok for a competition. You are so lucky not getting any fruit fly my tommys get riddled with them.
I dont grow any more in the Summer, even with baits. I am hoping they will leave my Nectarines alone,
this spring as I have a few on the trees this year, Good luck with your competition..Cheers..MM.