All airlines leaving Australia require that your checked baggage dimensions do not exceed 158 cm (height + width + length).
For weight restrictions there is usually a 20kg limit per bag, although some airlines have a 25kg limit. You can combine your weight with someone else in your group but the weight must not exceed 32kg per bag.
Current weight restrictions for the major airlines are:
Qantas 20kg
British Airways 20kg
Singapore Airlines 20kg
To increase your weight allowance you can try and join the loyalty programmes or if you can afford upgrade your seat as business class usually recieve an extra 10kg.