Thankyou for the suggestions and links. I have tried using ars (!) powder and also another powder that is sold in the paint section of Homepro. - I've also tried boiling water, soapy water and mossie spray!

All the above work fine on ants when they're out in the open. Our problem is that we rarely actually see them, instead what we see every morning is a series of perhaps a half dozen small piles of excavated soil, a couple of inches high, all within the same 3x3 metre area of the lawn. The only thing that has been effective so far is mossie spray, sprayed directly at the piles of dirt and surrounding areas, that keeps the ants quiet for about three days before they start to come back again but it also kills the grass where the spray has been concentrated. The two reasons we're having such low success with all of this is the fact it rains every day and washes away the powder - it's also really hard to know where to put the powder since the dirt mounds are rarely in the same place twice. Borax is the next remedy on my list so we'll see how that works out.