He defended his decision - which was taken during the pandemic - as "necessary" in "extraordinary times".
Mr Morrison is currently the member of parliament for Cook in New South Wales.
The revelations caused uproar among the public and his colleagues, with one calling his behaviour "dictatorial".
Mr Morrison became joint minister for the health portfolio from March 2020, and for finance, treasury, home affairs and resources from May 2021.
In a news conference on Wednesday, he defended his decision by saying he was "acting in the national interest in a crisis" in case a minister was incapacitated with Covid-19.
"They were put in there as a safeguard as a 'break-glass-in-case-of-emergency' and as a result, thankfully, we didn't need to break the glass," he said.
Mr Morrison added that he had never acted as minister despite being secretly sworn into those portfolios.
"I did not instruct any department that I was to have jurisdiction for carriage of any of the issues that the ministers were dealing with on a day-to-day basis," he said.
When queried about why he had not disclosed the expansion of portfolios to cabinet or to the broader public, he argued there was a risk the powers he took on could have been misconstrued.
"I think there was a great risk that... those powers could be misinterpreted and misunderstood, which would have caused unnecessary angst in the middle of a pandemic," he said.
The long-time politician has come under intense criticism in recent days from former colleagues - including his own party members - and the broader public after revelations broke about his expanded powers.
Current treasurer Jim Chalmers said Mr Morrison had "dictatorial tendencies", and called on Coalition leader Peter Dutton to condemn his actions.
"Scott Morrison was evasive, he was defensive, he was passive aggressive and of course he was self-serving, so at least he was true to himself today," Mr Albanese said.
The news came to light after two News Corp journalists wrote that Scott Morrison had assigned himself those roles during his leadership, in a recently-published book about his government's response to the pandemic.
Some ministers - including Mathias Cormann who was the finance minister at the time - were reportedly unaware they were sharing portfolios with Mr Morrison.
