As soon as they have a bit of fluid in the day, or their weight reaches about 4.5kg, the newborn nappies tended to leak, and I went up to the next size - infant. I find huggies not very absorbant in the top 20% of their weight category, so just keep that in mind!
Also, dont forget there are boys and girls ones from infant up, so if you dont know what you are having, you might be stocking up the wrong ones!
I guess newborns use about 12 nappies a day?