Put simply, if you want to replace the currency in 2026, start doing it in 2016.

The existing notes are already difficult to counterfeit, though it has been achieved on occasion; this is primarily a defensive move by the Australian Reserve Bank to keep the goalposts shifting under the feet of would-be counterfeiters.

Banknotes have a limited lifespan, typically between 3 and 10 years depending on the handling characteristics of the denomination. At some point, old designs exit common circulation. At first, retailers will begin treating them with greater suspicion, then eventually stop accepting them even though they technically remain legal tender. Banks will still redeem old Australian banknotes at face value.

If a large number of new looking older bills start showing up it'll raise a lot of eyebrows so you want to get a better bill out so you can start getting the older easier to counterfeit bills out of circulation before counterfeiters figure out how to make them. It doesn't solve the problem for small transactions in whichever country since you can pass a couple older bills at a time. It does help for large cash transfers to other countries and for USD, Euro, Pound sterling, Yen, Franc, and Yuan because there's foreign reserves to think about. A large amount of US currency is held by foreign governments and banks as a foreign exchange reserve. If it was ever easy to counterfeit the latest bill a lot of the faith these foreign institutions hold in the bills legitimacy would fall apart.

Australia is pretty good at removing old notes from circulation. After a decade or so it becomes difficult to pass old notes at shops without being scrutinised very closely.

Unlike in the US, in many countries notes are not valid for all time. After an expiry date they must be exchanged at a bank for a new note.