The team found both the platypus and echidna produce a long-lasting form of the hormone glucagon-like peptide-1 (GLP-1).
GLP-1 is normally secreted in the gut of both humans and animals, stimulating the release of insulin to lower blood glucose levels.
But GLP-1 typically degrades within minutes.
Lead researcher Frank Grutzner said his team was surprised to find the hormone was produced not only in the platypus' gut, but also in its venom.
"We've found that GLP-1 is degraded in monotremes [platypus, echidna] by a completely different mechanism," Professor Grutzner said.
"Further analysis of the genetics of monotremes reveals that there seems to be a kind of molecular warfare going on between the function of GLP-1, which is produced in the gut, but also surprisingly in their venom."
The platypus has spurs on its hind limbs for delivering venom to its competitors during mating season.
Associate Professor Briony Forbes said the two different functions of GLP-1 in the platypus — in the gut as a regulator of blood glucose, and in the venom to fend off other males — have seen the hormone evolve.
"The function in venom has most likely triggered the evolution of a stable form of GLP-1 in monotremes," she said.
Excitingly, stable GLP-1 molecules are highly desirable as potential type 2 diabetes treatments."
Professor Grutzner said GLP-1 was also discovered in the venom of echidnas, but unlike the platypus, they do no have a spur to deliver the poisonous fluid.
He said more research was needed to find how the treatment would work in humans.
"I think the next step is really to do some experiments in an established medical animal system like the mouse for example, and study how that variant is working in that mouse model and take it from there."
(Researchers found the platypus produces a long-lasting form of the GLP-1 hormone.)