Nice find, should make a great water bath for a tall stockpot shaped vat, especially with that drain!
The vat will float if you partially fill with water first, then will sink as you fill it with milk. Even then the weight of the stockpot shouldn't be too great and should be able to rest it on the stockpot handles if they are robust.
I don't know what stores you can order from in Oz but I ordered my heavy duty
38 liter stockpot vat with heavy duty handles from a store in US called
Instawares.com, (just surf to Kitchen > Cookware > Stockpots > Stainless Steel) may give you some ideas on sizes available to fill that barrel.
I can lift the whole pot from the two handles with ~32 liters in it and the handles were rock solid (although I was a little wobbly
).