Most rejections give one of five reasons: already in scope, no written instruction, late notice, cost not proven or your own error. Each has an answer if your records support it.
How it flows
Reply in writing and cite the clause, the instruction and the cost record. Keep the tone factual.
Follow the contract's escalation steps in order, such as a senior meeting. Across Australia, security of payment law gives a short window to apply for adjudication after a payment schedule, and the window differs by state, so diary the date. For example, Queensland usually allows 30 business days.
Check the contract before you stop work. Stopping can be a breach.
Ask for the reason in writing, test it against the contract, strengthen your proof and escalate under the dispute steps. Watch the security of payment deadlines the whole time.
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