Australian Standard Sizing is

Age Premature Newborn 0-3 mths 3-6 mths 6-12 mths 12-18 mths 18-24 mths 24-36 mths 36-48 mths
Size 00000 0000 000 00 0 1 2 3 4
Weight (kg) 2 3 6 8 10 12 - - -
Height (cm) 50 56 62 68 76 84 92 100 108

However each baby clothes manufacturer has different sizing (just like with womens' clothing).

Some, such as Frugi (designed to be worn over cloth nappies) tend to be more generous in their sizing.

Please select the appropriate manufacturer from the menu to find the correct size chart for the item that you are interested in.