The letter price rise has been known for some time, and it's been 4 years since the last price hike. Coincidentally I bought a 20 pack of 2015 Wildflower stamps from an Australia Post vending machine just yesterday (the last price hike was in 2016 but the $1 stamps were issued prior to that to make up for demand).

As for the parcel increases, that's just Australia Post being what it truly is, a privately-owned for-profit entity that only shares its trade name with the government mail service. Throw in a near-monopoly status and they can do whatever they want without restraint; not many other parcel/courier/freight companies let people walk in off the street, plonk $8 or so down on the counter and send something across Australia without a business account or sending in bulk or per tonne (and even if they do, it's usually stupidly expensive even on very small items). Even Mailman is just a deal between Officeworks and Fastway. But yes, everyone knows that it shouldn't cost $9 to send a 40 gram parcel from one suburb to the next just because it's thicker than 2cm. Just thinking of it, imagine how much more money the letters service would make if large letters could be 3cm thick. People might even be able to send actual documents through the mail service! Currently it's impossible to send a >2cm thick book, a newspaper or a small ring binder full of paper documents through the mail service since they are inexplicably not counted as large letters, or as documents for that matter, thus the parcel service takes over. Given that there is no such thing as media mail under Australia Post (unlike for example USPS) there is no provision for actual documents (paper, digital media e.g. CDs/SD cards/USB sticks/hard drives, audio/video recordings etc.) to be sent as mail if the items are over 2cm thick. Australia Post is constantly saying that no-one is using the mail service anymore, is it any wonder when almost everything has to go as a parcel?