We gave a small lawn area about 10m x 10m and its got infested with black ants in the last couple of weeks or so.
I'd live with them if they had different habit maybe but theyre making quite large holes everywhere and bringing lots of stuff up from underground and spreading it around and I think they're just going to make typical bush ant heaps and dead ground out of the whole thing.
What's the best treatment?
First of all try granulated Ant Killa by Richgro, available here in the Garden Shop. Just sprinkle it wherever you see ant hills.
An alternative is an application spray of Baythroid Advanced Insect Killer for Lawns by Yates. But you'll need to take great care as this is a strong chemical and you will need to rope off the area sprayed from children and pets for a few days. Also do not let it get onto any close-by flowering plants as it can affect bees.