The attached link will give you a bit of extra information about the weights at which horses run, especially weight for age.

http://www.australian-racing.net.au/rules/rules24.htm

The month along the top is the month of the race, so as an example a 3 year old racing in March over 1200m carries 55kg. These are male weights, anyone can correct me I think you'll find the girls carry 2.5kgs less.

Hope it helps.