what legal effect does this clause have in a sponsorship agreement between an modelling agency that represents a model and has been given authority by the model to act on the models behalf to go into an agreement with a sponsoring company.

' This agreement supersedes all prior agreements, arrangements and undertakings between the parties and constitutes the entire agreement between the parties relating to the Sponsorship. No addition to or modification of any provision of this agreement will be binding upon the parties unless made by written instrument signed by a duly authorised representative of the party.

with reference to Australian law please