You got fire ants down under?
What do you use to kill them with?
If its acephate or Orthoboric acid it will work on your orchids with no harm to the plants.
Its sold in the USA as 'Amdro Fire Ant Killer" Orthene
If you can find the equivalent there down under your roaches are all dead meat
It also kills most other orchids pest from scale, mealy bugs and aphids. It is well known to those who own orchid greenhouses to be safe.
Boric acid was dumped over Reactor 4 of the Chernobyl Nuclear Power Plant after its meltdown to prevent another reaction from occurring. Boric acid even keeps nuclear fission down. Whats a cockroach!!!