Why is the Australian Government financing the stimulus via debt rather than the RBA? The Australian Government is engaging in quantitative easing for the first time. The RBA is creating money that is being used to buy bonds on the secondary market so that there is enough liquidity for banks and financial institutions to buy new bonds to fund the covid stimulus measures.

Why are financial institutions necessary as an intermediate step? Why not fund the measures directly from the RBA without introducing additional debt (and the associated interest)?

I'd also be curious to know if Modern Monetary Theory has a different answer to this question than conventional economics.
As noted in comments, the reason why the RBA acted the way it did most likely comes down to its legal framework. Before the 2020 crisis, very few countries allowed their central bank to directly fund the fiscal arm of the government, which I will call the Treasury. I had often seen Canada cited as the only developed country that allowed the central bank to buy government bonds at auction (I am unsure that no other example exists). I think such financing was allowed in the 1945-1970 period in more places.

I know very little about the legal framework the RBA operates under. I will instead just discuss the economic effects.

The Modern Monetary Theory (MMT) view is that the addition of banks into the loop does not matter. Having the Treasury issue bonds that are just bought back bu the central bank - which is owned by the Treasury - has exactly the same economic effect as the Treasury directly borrowing from the central bank.

If we look at the mathematical models used by other economic theories, they say the same thing, since the final balance sheets are the same. The usual description of this is that this is “money financed deficits.” However, the argument seems to be that the difference is that behaviour of the fiscal arm will change, and so there is a greater bias towards inflationary policies.