Bergstein gumboots offical size guide. The length in centimetres is the maximum foot length for the corresponding size.

EU  AU  Length
21 4.5 14.5
22 5.5 15
23 6 15.5
24 7 16
25 7.5 17
26 8.5 17.5
27 9.5 18
28 10 18.5
29 11 19
30 11.5 20