NSW ran out of combinations of 3 letter/3 number plates in late 2004 and replaced the general issue plates with 2 letter/2 number/2 letter plates, issued numerically, ie AA 00 AA then AA 01 AA through to AA 99 AA then AA 01 AB etc. Also since early 1990's NSW plates were available in slimline format as 3 letter/2 number/1 letter ie AAA 00 A

Originally NSW had all numerical plates, starting at 1, but they were replaced with 2 letter/3 number plates. These were again replaced with 3 letter/3 numbers about 1951. At that time Australian States were given certain letters to start their plates. NSW A-F with F reserved for haevy vehicles, Victoria G-L, Queensland N-Q (with Q p[lates used on State Government owned vehicles), South Australia R-T (starting from 1967), Western Australia U-W, Australian Capital Territory Y, and Northern Territory all numerals. Tasmania used 2/letter/4 numbers from 1970. Plates starting with Z were reserved for Federal Government vehicles. After these letter combinations ran out the states started doing their own thing. NSW continued to issue plates alpha-numerically, Queensland started using 3 number/3letter combinations WA changed over to 1 number/two letter/3 numbers.
Of course there were also special combinations available at different times.