Isn't that essentially the point?a US Army Armored Brigade Combat Team is far removed from what the Australian Army is intending to develop.
The army if there is no budgetary concerns would build as @Takao says "What the army needs" which would look a lot more like a US Brigade Combat Team than not (Not saying Australia would develop an Armoured Brigade). For example, if you go off @Takao's calculations on tanks you get to 70 tanks per brigade. That isn't too far off an US Armored BCT which has ~87 (If i count correctly), bit less but not surprising as Australia wouldn't be going full armoured brigade but more combined arms oriented.
My previous post was probably poorly phrased and sort of halfway between two thoughts but what I meant is if Australia was to develop a brigade based on "What the army needs" it would be structured slightly different compared to how the US structures their brigades but not too different that the IFV counts would be massively different.
As the Army does have budgetary concerns they are developing something very different. If you compare this "Ideal" brigade structure to what the Army is developing you essentially have to keep cutting capability to get down to 450 IFVs, let alone 300.
For example, by cutting a battalion from all 3 brigades (2 in each instead of 3). Then only one of them is mechanized. Then replace some of the variants with less capable but cheaper vehicles etc