Males have a dense tuft of hairs on the front of their long jaws.
In the described male and female, the head and thorax are dark reddish brown. The legs have darker rings and the abdomen is mottled greyish brown and beige.
Austrarchaea harmsi occurs in leaf litter accumulated above the ground. Juveniles also occur on low branches in rainforest, extending its documented habitat into vegetation-supported litter and branch surfaces.
In May 2010, a male was collected from elevated litter at 959 metres near Dandabah, and a female at 1,030 metres on the same day. A juvenile had also been collected from a low-hanging bunya pine branch in 2005.
On 31 December 2020, a female, a male and a juvenile were collected at separate suspended-litter sites in the same national park. Repeated observations document these aboveground accumulations of dead leaves as places occupied by the species.
An extinction-risk assessment has not been added yet.
The 2011 revision described the species as narrowly distributed but locally abundant in the park. Park rules protect dead plant material and fallen timber, and the authority identifies summer fire risk. Surveys could track the rainforest, suspended-litter conditions and occurrence at known sites.