Does a cash job with no invoice cause problems?

The offer is usually framed as saving the GST. What it actually costs you is the paper trail, and the paper trail is what every subsequent protection depends on.

What you give up

• Proof of who performed the work, which every warranty and tribunal claim starts from.
• Proof of what was agreed, so a scope dispute becomes your word against theirs.
• Evidence for an insurance claim if the work later causes damage.
• Records for a future purchaser, and in some states a disclosure obligation you cannot satisfy.
• For an investment property, the deduction or the addition to the cost base.

What an invoice should contain

• The business name, the ABN, and contact details.
• The licence number where the work is licensed.
• A description of the work performed, not just a total.
• The date, the amount, and GST shown separately if they are registered.

What it signals

A contractor who will only work off the books is telling you something about how they will handle a defect claim, a warranty and an insurer's questions. Insurance and licensing are also administrative obligations, and a business avoiding one is often avoiding the others.