Printing banknotes is just like any other mass manufacturing process – the machines work perfectly 99.99% of the time, but mistakes sometimes happen.
Misprinted banknotes were always destroyed immediately (or at least they should have been), which would leave a standard bundle of 100 notes one or two short.
In Australia between 1948 and 1972, replacement notes were used to ensure complete bundles could be issued.
These replacement notes all have one feature in common – the last digit in the serial number was always replaced with an asterisk “*” (referred to by collectors as a star). This is why they are called "star replacement note", “starnote" or just “star".)
Decimal star notes printed between 1966 and 1972 all have serial numbers that start with the letter “Z”.
A star replacement note was used to replace a note that was misprinted during the printing process - they are incredibly rare, and are keenly sought by experienced banknote collectors.
Note Printing Australia has always had an excellent quality control system in place, so one star replacement note would only ever be seen for between every 200 to 1,000 standard notes back when they were in circulation - this is one in every two to ten bundles of regular notes.
Paper decimal star notes are generally worth at least a few hundred dollars, and in mint Uncirculated condition, can be worth many thousands of dollars.