The experts also disputed a claim presented by tech company Wing that birds would adapt to its drones, noting that magpies continued to swoop cyclists and pedestrians despite years of coexistence.

Documents released under freedom of information have revealed the government's conservation research unit identified risks to local wildlife from drones.

Wing said it had already adjusted its operation after receiving the unit's advice late last year, and "strongly believed" its planned permanent operation in Canberra's north would cause a "minimal environmental impact".

Opponents of Wing's Bonython trial repeatedly raised concerns about the threat of drones to local wildlife, claiming bird numbers in the quiet Tuggeranong suburb declined following the aircraft's arrival.

The US-based tech company commissioned an environmental consultant to study the potential impacts of its drones on local wildlife during the controversial trial.

Wing presented the report to the government last October, but has refused to release it to the public, citing commercial confidentiality.

But the conservation unit's draft peer review of the study has been released - and it delivers a strong rebuke to a number of the consultant's findings.

According to the peer review, the consultant determined the drones were unlikely to cause a "significant impact" to birds, meaning it was not necessary to establish a bird monitoring program.

The conservation unit said that conclusion was unfounded.

"There is insufficient information presented in this review, or likely to exist at present, to assume an insignificant impact of drones on ACT or Commonwealth-listed threatened species, or even general wildlife," it stated.

"In fact, the data which is available would suggest some impact - possibly a significant impact on a number of listed species - is likely. The assumption in the review that there is little significant birdlife within the urban footprint of Canberra is known to be invalid."

The unit's review also disputed an assumption that species would adapt to the presence of drones.

"[The assumption] is inappropriate given the migratory nature of many of the highest-risk species (eg superb parrot and little eagle); and also the little evidence demonstrating 'habituation' of local species to other anthropogenic activities.

"For example, Australian magpies and masked lapwings have not ceased to consider cyclists and pedestrians and pedestrians as threats to their urban territories, and so continue to expend energy - and risk injury - by swooping intruders."

The unit agreed with the consultant's recommendation to establish no-fly zone along the Murrumbidgee corridor, but said the Canberra Nature Park and other "known habitat areas for threatened species" should also be declared off limits.

The unit delivered three further recommendations, including that any future expansion of the technology "only be undertaken when evidence is available of no or low significant risks to local-listed threatened species".

The drones should also be fitted with cameras to monitor "wildlife incidents", including attacks by birds, during trials, according to the conservation unit.

The government presented the draft findings to Wing in late November 2018, according to an email circulated among officials at the government's environment, planning and sustainable development directorate.

The email, marked "commercial in confidence" and "sensitive", stated that as the findings were in draft form, they did not represent the views of the directorate nor the ACT government.

The Canberra Times asked the directorate whether the conservation unit had completed a final peer review of the consultant's bird study, and whether it had delivered the same findings - including that expansion of drones to others parts of Canberra be halted until threats to wildlife were addressed.

A spokesman said the directorate could not provide a copy of the fauna study as it was subject to a freedom of information process. Wing had commissioned NGH Environmental to do the study. "It is the conservation research unit’s role within the ACT government to provide advice on conservation matters as part of its normal business, the spokesman said.

The directorate's deputy-director general, Geoffrey Rutledge, told the ACT Assembly's drone inquiry this month that the science on the impact of drones on birds was "unclear".

"Birds are very adaptable, and they work with the growing human population of Canberra," Mr Rutledge told the inquiry. "The impact is unclear, but it is an emerging area."

A Wing spokeswoman said it had responded to a number of recommendations from the unit's review, including increasing the average cruising height during delivery flights from 20 to 30 metres.

The spokeswoman said the Google-linked company was committed to being "good stewards of the environment" as it pushed ahead with an expansion into Gungahlin in the coming months.

"In Mitchell, we strongly believe that there will be minimal environmental impact to local birds," she said.

“In the more than 70,000 test flights we've performed around the world, we haven’t observed or have any data that indicates we've struck a bird. The only interaction with surrounding birds is the rare instance in which a bird (magpie) has swooped at our drone, but not made direct contact."