The five stars, of course, represent the constellation Crux, or the Southern Cross. It appears on the Australian flag, the formal description of which is quite precise as to the size, location and number of points each star must have. On this flag, the four stars at the ends of the cross are equal-size and have 7 points, while the fifth has 5 points. So if the stars are intended to be symbolic of the Australian flag, then the depiction on the $20, 1988 50¢ and 2000 50¢ are "correct".
As you suspected, the coat of arms and flag of Victoria predates Federation. Indeed, there was some concern at the time that the new flag of Australia looked too much like the flag of Victoria. However, in the formal description of this flag, the stars are different shapes and sizes, in the manner depicted on the CoF 2001 50¢ for Victoria. The "Victorian pattern" is actually more astronomically correct; the brighter stars in the sky are larger and given more points on this flag.
The 2001 20¢ coins were designed by school students, who were probably not well educated in vexillology before submitting their designs. The stars on the South Australian coin presumably represent the actual stars in the sky, rather than those on the flag. The mint was also keen to not "re-interpret" the student's handiwork.
So to conclude: you're both right. The stars on the Australian coins are correct for Australia, the stars on the Victorian coin are correct for Victoria, but the stars on the South Australian might be considered an "error".
As you suspected, the coat of arms and flag of Victoria predates Federation. Indeed, there was some concern at the time that the new flag of Australia looked too much like the flag of Victoria. However, in the formal description of this flag, the stars are different shapes and sizes, in the manner depicted on the CoF 2001 50¢ for Victoria. The "Victorian pattern" is actually more astronomically correct; the brighter stars in the sky are larger and given more points on this flag.
The 2001 20¢ coins were designed by school students, who were probably not well educated in vexillology before submitting their designs. The stars on the South Australian coin presumably represent the actual stars in the sky, rather than those on the flag. The mint was also keen to not "re-interpret" the student's handiwork.
So to conclude: you're both right. The stars on the Australian coins are correct for Australia, the stars on the Victorian coin are correct for Victoria, but the stars on the South Australian might be considered an "error".
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis



















