Female: Mid-sized mosquitoes of mottled dark appearance; proboscis dark scaled but with extensive pale area on the underside; scutum generally covered with golden and dark narrow scales but with some larger pale towards rear; wings with extensive mottling of broad pale scales; hind legs with femur, tibia and first tarsus mottled, all tarsi banded (although fifth may be all dark); abdominal tergites dark with pale lateral patches and basal bands which may not be complete and there may be some mottling on terminal segments, sternites pale scaled with some dark scales in mottling or apical bands or lateral patches.

Adult females of Ae. theobaldi can be confused with Ae. eidsvoldensis which have the tergal bands produced into a median triangle; other species with mottled proboscis, wings and legs such as Ae. flavifrons (blotch on wing membrane), and Ae. vigilax (fewer pale scales on wing, and scales are narrow like the dark ones not broad) can be readily separated; Ae. normanensis can appear similar but generally has darker proboscis.

Adults may become active in spring and be apparent throughout the year in warmer areas providing natural flooding or irrigation promotes an egg hatch; day-biting is usually apparent as the species readily attacks humans and other animals but they will bite also in the evening and at night.

May be a major pest following extensive rain or flooding in and can disperse for many kilometers when there are major larval populations; the species has been shown to be able to encephalitis viruses in laboratory studies, but there is no information as to any role in transmission of human disease.