Put it on anything paying better than 6 to 1 odds at fixed odds.
Don't put it on the totaliser or you will find it comes in a little not long after you get your bet on.
Then buy Commercial properties with the untaxed winnings. Too easy.
Putting jokes aside apart from that risky proposition I suspect 300k is only a stepping stone to retirement. It is not of itself going to be enough, but the first 300k is the hardest to save so if you have that much you are well past half way to a million. Or better if you have been gifted it you have a massive 10 year plus start on someone who has to start from scratch with an above median job.
Say you can save 20k from your PAYG income per annum.
Now you get another 20k in income off your investments total $40,000.00 p.a. saved so your saving rate is double what it was when you first started investing and only had your regular PAYG income savings.
Are roulette odds good enough ?