Excess luggage weight for internal flights in China

I will be travelling from Adelaide, South Australia to Beijing. I am aware that I am allowed 30 kg baggage weight for international flights. However once we are in Beijing we will be flying some domestic flights to to other internal destinations in China. These flights require a 20 kg max luggage weight. What happens with my luggage as it will still be 30 kg weight from the international flight? Can you please advise the excess baggage weight costs for luggage over 20 kg? Thank you, Lorraine