Hand-held lasers are increasingly being used against Australian Defence Force assets, with military insiders blaming small Chinese maritime militia vessels for the dangerous tactic.

Key points: Pilots were targeted on a night-time flight over the South China Sea earlier this year

Pilots were targeted on a night-time flight over the South China Sea earlier this year But Defence said no "military-grade lasers" were used against Australian Navy or Air Force units in the past year

But Defence said no "military-grade lasers" were used against Australian Navy or Air Force units in the past year One academic suspects fishing boats are being used to make things difficult for foreign militaries

Earlier this year, the Defence Department confirmed military helicopters were targeted on a night-time flight in the hotly contested South China Sea.

The pilots were temporarily forced to return to their ship for medical check-ups following the incident, which occurred in May during the Indo-Pacific Endeavour 2019 exercise.

Now Defence has revealed the use of lasers from fishing vessels appears to be on the rise, particularly in congested waters.

"ADF personnel operating on vessels and aircraft have observed, in recent years, an increase in the use of low-strength hand-held lasers by some fishing vessels," the Defence Department said.

"The reason for fishing vessels using the lasers is unknown, but this may have been to draw attention to their presence in congested waterways."

In a formal response to questions from Labor senator Penny Wong, Defence said it was important to "distinguish between the use of low-strength lasers by fishing boats and the more powerful military-grade laser devices employed by military, coast guard and some civilian vessels".

"Australia would view reports of the more powerful military-grade laser devices being used against civilian and military vessels as deeply concerning and potentially dangerous," she said.

"There have been no reports of military-grade lasers being used against Royal Australian Navy or Royal Australian Air Force units in the last 12 months," the Department confirmed.

Chinese People's Liberation Army Navy personnel in a South China Sea military display last year. ( Reuters )

Senior Defence figures say Chinese-flagged fishing vessels operating as part of Beijing's maritime militia are known to employ the tactic against military aircraft that transit through the South China Sea.

Doctor Euan Graham from Latrobe University was embedded on an Australian warship during Indo-Pacific Endeavour 2019 and believes Chinese fishing vessels were to blame for the laser attacks.

"I think it's consistent with a long-term objective on the part of China to make life difficult for foreign militaries operating in and over the South China Sea, not necessarily by force-on-force encounters but rather by making use of indirect means including the so-called 'maritime militia'," he said.

"It's highly dangerous because anything that blinds the sight of pilots even temporarily will incapacitate them and increase the chance of a collision or an emergency landing."