that's preety bad luck mate, not good to hear. :(
I had a stack of problems last year, lost 5 or 6 hives to massive swarms of bull ants
There is only one permement remedy for bull ants, that worked for me. here it is
Go to bunnings and get a product called "david grays ant and termite spray". its a concentrate. small white box. nasty stuff! get one of those pressure sprayer packs while your there. dilute it 30ml per ltr then follow the ants back to their nest and apply to the point of overflow down each hole entrance. follow it up with another dose 2-3 days later....no more bull ants....
eco 1 ants 0
make sure you have a really good look for all the nests...At at one site there was a cluster about 80 meters away that were harassing my hives....
Cinnamon powder (i'm talk-en heaps of it) also works well around the entrances and hives stands (use it around the hives where you absolutely do not what this other nasty stuff). seems to "deter" ants.
best of luck