If you cannot resolve the problem by speaking to the client directly, send them a letter or email, formally outlining the issue and requesting a response.
By putting your concerns in writing, you will have a record of your discussions. This can be shown to a third party if you choose to take the dispute further later on, and demonstrates that you made a reasonable attempt to resolve the problem yourself.
You should use registered mail if you are sending your correspondence by post.
If you need further advice about resolving the issue, you can contact the Building Information Line on 1300 55 75 59 between 9am and 5pm, Monday to Friday (except public holidays).