A things worth keeping in mind from my POV regarding the Australian-US defence relationship.
Both Australia and the US will (naturally, IMO) act in their respective national interests, which is why sometimes one nation or the other will do something the other does not want.
However, the two nations are very much friends, allies, and partners. This means that things will be done for the benefit of the other more often than not.
Regarding sales and purchases of defence material, the relationship as GF and Assail mentioned is very good. IMO this is because from a US POV, such sales to Australia are not just commercial/industrial activities.
Using the future sub programme as an example, the French, Germans, and Swedes are all very interested in winning the contract for the future RAN sub, because it will mean an increase in some exports, an expansion of market share, and potentially open new export markets to their nations' respective goods. The success or failure of new RAN subs will not potentially impact the national security of those nations, or their armed forces. Apart from French Polynesia, none of the European contenders have either territories or forces in the Pacific. Meanwhile, the US does indeed have significant territories and forces in the Pacific. Given the degree which the US works with, and relies upon (or recognizes that it might want to/have to rely upon, especially in an emergency) Australia, then it is in the best interests of the US for Australia to have the best kit available, which is also able to seamlessly integrate alongside US forces.
An anecdotal joke I have heard here on DT and elsewhere is that the USN's 7th and 1/2 Fleet has another name, which is the RAN...
One of the other reasons why I suspect Australia can get so much support, is the degree of trust between the two nations. There are a few allied nations which have not (or at least have not been caught) conducted espionage operations against the US, the UK, Canada, and Australia are on that short list, and I suspect NZ is as well. Germany has conducted such operations, and I believe France has to an even greater degree. This is not just in terms of gov't-to-gov't operations, but also industrial espionage where businessmen have had their hotel rooms tossed and personal electronics stolen and/or copied.