I believe you need a receipt: “If you pay for anything for the scheme in cash, or get any cash for the scheme, you must keep a specific record of the amount of cash going in or out, and what it was for. Keep copies of all receipts or invoices for the cash payment.”
If you have a Strata Manager it would be worth discussing with them, if you don’t it would be a good idea to ensure that all compliance is being checked on contractors before engaging them as possibly not if cash in hand jobs are happening.