Q.reusing soil where tomatoes have wilted
I recently had a large tomato in a pot wilt suddenly. It had plenty of water,
Rather than throw out the soil I used it to top dress patch of grass. Now that grass patch has died. Would the same thing as wilted the tomatoes caused this?
Yes it is possible that a soil borne fungus disease affected the tomato plant and was transmitted to the grass. I suspect "fusarium wilt".
It is difficult to control, but a preventive approach is keeping the grass and other nearby plants as healthy as possible with good soil fertility and water management.
There is a biological fungicide specific for fusarium prevention, but it should not be considered as a cure for active infections, only for prevention of further infections.
Mycostop is available in the US, I'm not sure about Australia: