A series of investigations into alleged sexual crimes committed by US marines in and around Darwin have been quietly dropped by Australian and American authorities, ABC News can reveal.

That decision raises questions about whether Australia and the US are doing enough to investigate claims of sex crimes, and echoes decades of international concern about the cover-up of those crimes by US military personnel serving abroad.

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"I think that any sort of criminal activity against Australian citizens … should be dealt with appropriately by the law," said Flinders University sociologist Ben Wadham, a former Australian infantry soldier and military policer officer.

"And in this case I would argue the [Australian] Defence Force are actively attempting to keep it out of the public eye," Dr Wadham said, adding it was difficult to know how serious the cases were, given the lack of transparency.

Barack Obama and Julia Gillard address US and Australian troops at RAAF Base Darwin in November 2011. ( White House: Pete Souza )

Thousands of US marines have been based at Robertson Barracks outside Darwin since late 2011, when then-prime minister Julia Gillard and then-US president Barack Obama announced a plan to bolster military cooperation in Asia. The announcement was part of the Obama administration's strategic pivot to Asia.

ABC News has obtained half-a-dozen reports authored by the Australian Defence Force Investigative Service (ADFIS) between 2013 and 2015 via a freedom of information request. The reports include three cases of indecent assault and inappropriate behaviour by US marines.

The documents are heavily redacted but provide an insight into how allegations of sexual or indecent assault by US military personnel in Australia are treated.

In one report, the NT Police investigated two alleged indecent assaults dating from August 2013.

One incident occurred at a bar in Darwin's CBD. The other involved "unwanted sexual advances" in a Darwin hotel room.

NT Police interviewed the alleged victims and sent briefs of evidence to the NT Director of Public Prosecutions (DPP).

"The DPP advised NT Pol[ice] that based on the evidence available, there would be no reasonable prospects for a successful prosecution with either complainant," the ADF report stated.

'Jurisdictional issues' stalled investigation

Another report, from September 2014, outlined how ADFIS began an investigation into an allegation of inappropriate behaviour that stalled.

"All ADFIS action in this matter has ceased due to jurisdictional issues," the report stated.

A US marine rifleman on patrol during an exercise at Cultana Training Area, South Australia in July, 2016. ( US Marine Corps: Lance Cpl Osvaldo L. Ortega III )

When ABC News enquired about the "jurisdictional issues", a Defence spokesperson said: "The complainant chose not to make a criminal complaint, they elected for the US Army to deal with the matter in its entirety."

A spokesman for the US Marine Corps, Lieutenant-Colonel Curtis L Hill, said he could only identify one case from the ADFIS reports, the September 2014 matter, which he believed related to an allegation that a marine behaved inappropriately towards a gym employee.

"The subsequent investigation showed the allegation was exaggerated and there was no legal action taken," he said.

In another case, a US marine was accused of inappropriate behaviour during an exercise at a South Australian military training area on March 23, 2015.

That matter was not investigated because someone, presumably the alleged victim, "declined to make a formal complaint regarding the incident", the report states.

Military 'protective' of US-Australia relationship

Dr Wadham said it was possible Australian and US forces were more concerned about the military relationship than they were about properly investigating the allegations.

"This particular relationship is very important for the Australian Government and the US Government," he said.

"The military like to be very protective of that and try to keep that as clean as possible."

A Defence spokesperson denied that assertion. "A victim-centric approach is maintained throughout the investigative and follow-on phases, not any notion of protecting international relationships.

"In this case, the victim's wishes determined the level of Australian response. Defence has not treated the case to which you refer any differently to any others, and has cooperated with the relevant US authorities as they have handled their response."

One case not mentioned in the Defence police reports is that of US sailor Hugh Patrick Malone, who in November 2014 was found not guilty of the rape of a Darwin woman.

Concerns were expressed after the 2011 US marines announcement that Australia could experience the same issues endured by countries such as Japan, which has had a US military presence since the end of World War II.

Japan has experienced a slew of cases of rape, murder and other serious crimes committed by US military personnel since then.

In 2014, the Associated Press obtained records of more than 1,000 alleged US military sex crimes in Japan. The news agency's investigation found only about a third of the 244 military personnel who were punished served any jail time.

The Australian Defence Force has also endured claims that in the past it failed to properly investigate alleged sex crimes.

A public outcry over an incident in March 2011, during which a female ADFA cadet was filmed without her permission during consensual sex, led to a series of investigations into Defence's culture.

Investigators found sexual harassment and sex crimes were often not reported because of fear of victimisation, and that "pockets of poor leadership" allowed a culture that tolerated sexual harassment and sex crimes to exist.

'US marines are good neighbours'

The ADF defended its response to the assault accusations. "While instances of inappropriate behaviour are rare and inconsistent with the overall conduct of US marines in Darwin, the ADF and the US Marine Corps take allegations of misconduct very seriously," a spokesperson said, adding that civilian police had primary responsibility for investigating criminal allegations involving military personnel.

US marines compete in a shooting competition in Darwin. ( ADF )

When asked why none of the matters had led to prosecutions, they said: "There may be a number of reasons why a particular complaint does not proceed to a trial.

"These include where the victim does not wish for the matter to proceed, or where there is insufficient evidence to proceed."

Defence also failed to place the incidents on a public register of incident reports called Hot Issue Briefs (HIBs). The HIBs register was created as a means of increasing Defence's transparency in the wake of the ADFA Skype scandal.

The three incidents were not included on the HIBs register because publication may have hindered investigations by Defence or civilian authorities, the Defence Department said.

However, the HIBs web page notes Defence is able to create a HIB and release it once investigations have finished.

The ADF did not comment about why that did not occur.

The US Marine Corps also defended its behaviour.

"US marines are good neighbours who are engaged with and contribute to host communities," Lieutenant-Colonel Hill stated in an email.

"Allegations of misconduct are taken very seriously and investigated thoroughly by the command or agency exercising jurisdiction over the allegation. When allegations are substantiated, appropriate legal or administrative action is pursued."

In another case in the ADF reports, an Australian military member was accused of sexual assault in the US.

He faced no sanction after the US Army found there was "insufficient evidence" to criminally charge him, Defence told ABC News.