Does milk expand when frozen?

Yes it does. Milk is typically 80-90% water, and water most definitely expands when frozen.

Some information regarding water composition of milk:

https://en.wikipedia.org/wiki/Milk#Nutrition_-_comparison_by_animal_source

Many years ago when milk was delivered to your door, and came in glass bottles with a tinfoil cap, it was a common sight on winter mornings to see the lids pushed up by a white column of frozen milk if you didn't collect it quickly enough after delivery.

http://www.bugbitten.com/photos/Europe/jennilea/Let_it_Snow_Let_it_Snow_Let_it_Snow/88769-15162-3175033.html

PS - I grew up in Massachusetts, not Melbourne!

"Many years ago when milk was delivered to your door, and came in glass bottles with a tinfoil cap" - it still is, over here in the UK..... Our local dairy has only just changed over from the old wider necked bottles to the narrower necked little fat ones (and only because the manufacturer stopped making the older type). Delivery is by electric milk float.