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Family Sepsidae
Flies in this family are small and ant-like. Their wings are clear and usually have a small dark spot on wing near apex. They have the habit of continuously waving the wings when at rest. Adults usually found in swarm near the breeding places. Their larvae live mainly in mammal faeces.
Backyard.
Very small, about 5mm long.
A similar fly to yours is at http://www.brisbaneinsects.com/brisbane_... and is called an Ant Fly as opposed to black scavenger flies or ensign flies per Wikipedia.