A crane fly with a forked penis that seemingly evolved to walk rather than fly has been accidentally discovered by a scientist in New South Wales while undertaking routine work in Kosciuszko National Park.

The fly's unusual anatomy also includes tiny wings — measuring one tenth of its body length — and hairy long legs, and it is about 1.5 centimetres in length.

Professor Gunther Theischinger, who made the discovery, said he had never seen anything like it in his 50-year career studying aquatic insects.

Professor Theischinger believes the 'double-barrelled' penis could possibly compensate for a lack of functional wings. ( Supplied )

"The particular features of the inserting organ are not very normal in this group of flies, because usually it's a hose or pipe-like structure, [but] in that case it's a double-barrelled structure," he said.

"We haven't seen a very short-winged crane fly male in Australia before, and also the genitalia are very different from what we have seen in about 1,000 crane fly species.

"It doesn't even fit into any of the genera that are known from Australia, and even across the whole planet."

He said it was still not clear what advantages the bizarre-shaped penis would give the fly — named Minipteryx robusta, meaning tough, tiny wings.

"One possibility is that in the various position changes and twists during crane fly copulation the lack of functional wings may be a disadvantage," he said.

"The 'double-barrelled' penis could possibly compensate by giving the male additional strength to grasp the female."

Small wings show crane fly evolved for walking

Minipteryx robusta's wings measure only one tenth of its body length. ( Supplied )

Professor Theischinger also said the legs and smaller wings were an indication the fly had evolved for walking, rather than flying in a cold, windy, high-altitude environment.

He said wings were of little use in high altitudes and a cold climate because the rate of vibration required for flight usually could not be achieved.

"They cannot be wind-drifted away from their preferred, or liveable, habitat if they have no wings," he said.

Professor Theischinger said it is common for crane flies to be classed according to their features and, in particular, their genitalia.

"Crane flies are usually classified on the male anatomy or particulars of the male genitalia, but they are not simple like mammal genitalia," he said.

"They are combined, sclerotised structures that all serve to reach a good, kind of, fit while copulating."

He said his team has been looking for more of the new flies since May last year, but that the fly was "very inconspicuous and it is like you're looking for a needle in a haystack."

Professor Theischinger said it was a "remote possibility" a female fly would be found, but that the discovery showed unexpected scientific discoveries were still being made, "sometimes when you least expect it".

He has named more than 40 new genera and approximately 730 new species over the course of his career.