Austroaeschna sigma is a species of large dragonfly in the family Aeshnidae, known as the sigma darner. It inhabits the upper reaches of small mountain streams in New South Wales and south-eastern Queensland, Australia.

Austroaeschna sigma is a very dark dragonfly with pale markings. It appears similar to the multi-spotted darner, Austroaeschna multipunctata, which inhabits mountain streams in southern New South Wales and Victoria.

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