Prince Philip was legally a knight for more than a fortnight before Tony Abbott revealed his captain's call to the country on Australia Day.
The prime minister's office first sought advice about giving the Queen's husband a knighthood in November 2014, Department of Prime Minister and Cabinet officials confirmed on Monday.
They also revealed the Letters Patent for the Order of Australia had to be amended because the Duke of Edinburgh was not an Australian citizen, making him an "honorary" knight only.
It was also necessary to place Prince Philip ahead of Prince Charles in the order of precedence.
Mr Abbott sent a cover letter to the Queen with the amended Letters Patent in mid-December.
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The Queen agreed to the recommendation to knight her husband on January 7, 2015.
Cabinet minister Eric Abetz told a Senate estimates hearing on Monday that he only found out about the knighthood when it was announced publicly on January 26.
Mr Abbott, when asked by Labor during question time why he didn't tell his senior colleagues in November of the knighthood, said Order of Australia honours did not go to cabinet.
"Simple as that," he said.
The Order of Australia Council, and not the prime minister of the day, will now have sole responsibility for creating knights and dames.
