Mr Morrison has been criticised and censured for secretly taking on six portfolios when he was prime minister.
But until now it has been unclear why he took on the treasury portfolio, held by Josh Frydenberg.
Documents released under freedom of information laws show the treasury swearing-in related to the Foreign Investment Review Board (FIRB).
"We are advised the Treasury swearing relates to FIRB," the email dated April 21, 2021, read.
The role of the FIRB is to examine proposed investments in Australia that are subject to the Foreign Acquisitions and Takeovers Act, and to make recommendations to the treasurer on the proposals.
The same email, written by senior Department of Prime Minister and Cabinet official John Reid and circulated to senior colleagues, showed Mr Morrison had sought home affairs in relation to "decisions on citizenship loss".
Mr Morrison told parliament on November 30 that the reason he had taken on the treasury role "as a dormant redundancy for decisions that were not subject to cabinet oversight, was to be able to take swift action, if necessary, in the national interest in a time when Australia's interests were under constant threat".
"I consider that these decisions, in hindsight, were unnecessary and that insufficient consideration was given to these decisions at the time, including non-disclosure," he told parliament.
A report by former High Court judge Virginia Bell triggered changes to the transparency of ministerial appointments and led to Mr Morrison becoming the first former prime minister to be censured by the parliament.
Mr Morrison has admitted to using his extra powers only once - as resources minister in overturning Asset Energy's PEP11 gas exploration permit off the NSW coast.
The then resources minister Keith Pitt was in favour of the project.
A separate set of documents also released under FOI laws showed the National Offshore Petroleum Titles Administration (NOPTA) told Mr Morrison it had advised the joint Commonwealth-NSW authority to approve the permit extension and variation.
It noted that if Mr Morrison, as resources minister, sought to refuse the application he should trigger a "procedural fairness step".
This step should involve writing to NOPTA notifying of the joint authority's intention to refuse and "afford the applicants 30 days to respond".
"Community opposition and environmental concerns are matters you may have regard to, and you can determine the weight to give them," the advice said.
"However, public interest grounds cannot be the sole reason for refusing this request."
Asset Energy has sought a judicial review of the decision, saying Mr Morrison breached the requirements of procedural fairness and that he was not validly appointed as the responsible minister of the joint authority.
A Federal Court hearing is listed for March.
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