20Kg Checked in is a standard economy sized baggage allowance.
7Kg restriction for carry on luggage is a CASA requirement.
All you well informed PPRuNe'rs should know this.
With regards to the excess baggage, well it's clearly stated when you book the ticket. (not just in the fine print of the Terms and Conditions)
Blah, blah, blah it's the same old tired story we've heard before.