progress payment に正しい時点で GST を課す

Record GST at 15 percent with the date it took effect, 1 October 2010, so that anything valued before that date is computed at the rate that applied then rather than at today's. Keep it as one setting the whole system reads instead of a number typed into each template.

理由: A rate that lives in six templates is a rate that will be six different numbers the first time it changes, and the errors will be in the templates nobody uses often. Holding the effective date beside it is what lets an old valuation, a retrospective claim or a long-running contract be recomputed correctly instead of being quietly restated at the current rate.

Record two things off the contract: whether the contract price is stated inclusive or exclusive of GST, and the rule that decides when a progress payment becomes due, which on a construction contract is usually a number of working days after a claim is served or certified. Both belong on the contract record where the person raising claims can see them.

理由: An inclusive price misread as exclusive is a thirteen percent error on the whole contract, and it survives review because both readings produce a plausible number. The due date rule matters for a different reason: it is one of the three dates the time of supply test looks at, so a contract term is doing tax work whether or not anybody in the finance team has read it.

For each progress payment take the earliest of three dates: the date the payment becomes due under the contract, the date it is actually received, and the date an invoice is issued for it. That date decides the period the GST belongs to. Deal with the retention slice separately, because retention money is accounted for when it becomes payable rather than when it was deducted from the claim.

理由: This is where construction differs from ordinary trading and where the money is lost. On the invoice basis the tax can fall due in a period when the payer has not paid, so a slow client funds its own delay out of your account. Knowing the date in advance turns that into a planned outflow rather than a surprise in the week the return is filed.

Run the check over the documents you issue: the supplier's name and GST number, the date, a description of the work, the amount and the GST, and the recipient's details where the value requires them. Since 1 April 2023 this is taxable supply information rather than a document that has to be headed tax invoice, and it may be spread across more than one document as long as the set is complete.

理由: The obligation is now about information rather than about a form, which is easier to satisfy and easier to fail quietly, because a document that looks like every other invoice can be missing the one field that lets your customer claim the input tax. They discover it at their return, months later, and the correction lands on the relationship as well as on the paperwork.

Read the output tax the period carries against the input tax on what you bought, and read both against when the money actually moves. Record which accounting basis the business is on, because the invoice basis and the payments basis put the same job's tax in different periods, and the payments basis is only open to smaller registered persons.

理由: A construction business fails on cash rather than on margin, and GST is one of the largest predictable outflows it has. Read a month ahead it is a payment to plan for. Read on the due date it is the reason a subcontractor waits, and that has consequences under a different Act entirely.

File the claims, the taxable supply information, the credit notes and the workings behind each return together, project by project, and keep them for seven years. Keep the workings, not only the outputs, because a query is always about how a figure was arrived at rather than about what it was.

理由: Seven years is longer than most construction businesses keep their project folders and longer than most of the people who built the job stay in it. A query about a return from four years ago is answered in an hour by a business that filed the workings and in a fortnight by one that has to rebuild them from bank statements.