The ex-agent, known as Marcus, has previously claimed in ABC reports he warned ASIO about the father-son duo Sajid and Naveed Akram, six years before they committed the mass shooting which killed 15 people on December 14.
The Royal Commission into anti-Semitism and Social Cohesion wants to hear his evidence as it probes the terror attack.
In a letter to the ex-source's lawyers, the inquiry said it was happy for the correspondence to be provided to Australian officials as part of any application.
But it noted it would not be appropriate for the royal commission to assist him in any direct way to secure a visa.
"We remain eager to facilitate a practical way of receiving your client's proposed submission or taking a statement from him," the letter reads.
"The information that your client has outlined to us at an appropriate level of generality clearly and directly engages the royal commission's terms of reference."
Marcus has lived overseas since 2023, when ASIO withdrew its support for his permanent residency.
Due to the security nature of information, it can only be provided in person or at an Australian embassy overseas.
Lawyers for Marcus say he cannot access an Australian embassy and is concerned about security risks.
They argue the need for his engagement with the royal commission to be protected by Australian law, which doesn't apply overseas.
In a rejection letter from the Department of Home Affairs, officials said they were "not satisfied the applicant has a genuine intention to stay temporarily in Australia".
But the officer noted Marcus had previously travelled to Australia and had complied with the visa conditions which had been given "some positive weight".
Opposition home affairs spokesman Jonno Duniam said it was important the royal commission has access to all the information it needed.
"If the government are serious about ensuring the commission gets the right outcome and, in doing so, has access to all the information it needs, then it should explain why it won't grant a visa, or indeed find ways to grant one," he told ABC Radio on Thursday.
A number of crossbenchers have said the needs of the royal commission should come first.
The office of Home Affairs Minister Tony Burke was contacted for comment.
