Little is known about the narrowbody handfish, with just two specimens, obtained in 1986 and 1996 off the coast of Victoria, in existence.
Scientists were reviewing footage from a recent five-week trip on CSIRO vessel RV Investigator when they made the discovery.
CSIRO research technician Carlie Devine said the handfish was 292 metres deep on the edge of an underwater canyon near Flinders Island, off mainland Tasmania's northeast.
"Handfish are so elusive to find. To be able to see (one), particularly one at this particular depth, was pretty extraordinary," she said.
There are several species of handfish, so-called because they get around using their fins as "hands".
This includes the critically endangered spotted handfish, which lives at depths of 5-15m and the even rarer red handfish.
Spotted handfish were once common in southern Tasmania's Derwent estuary but experienced a severe decline in the 1980s.
Ms Devine said the image was likely of a narrowbody handfish because of the depth and other features, although confirmation couldn't occur without a specimen.
"It doesn't have any markings. It is very pale pink in colour. It has more of a robust upper body compared to other handfish," she said.
"Without collecting a handfish we can't really confirm what it is. But that's our best guess and I think it is a reasonable guess."
Ms Devine said it wasn't unusual to see a handfish by itself on the seafloor.
"They are often alone. When we do diving for our spotted handfish ... they are usually always by themselves," she said.
"They are very much lonely little creatures."Â
The preferred habitat and potential population of the narrowbody handfish remain unknown, Ms Devine said.
The discovery was part of a trip aiming to better understand how climate change is impacting the marine environment off southeast Australia.
Ms Devine said she was hopeful of another voyage to the area and further underwater surveillance next year.
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