Can I push back on amended payment terms before signing?

Yes. Whatever the draft says, the Construction Act already guarantees the machinery: a Due Date, a Final Date For Payment, notices with fixed deadlines, no pay when paid, and where the terms fall short the Scheme fills the gap. So you are never negotiating for those rights, only about the numbers hung on them. The negotiation is about the periods and the paperwork: a 45 or 60 day final date, application windows that skip a month if you miss a day by a day, notice deadlines shorter than the defaults, retention held longer than the job justifies. Push back in writing, briefly and specifically: name the clause, state the term you can hold, and give a commercial reason, cash flow on materials heavy packages being the usual honest one. Two or three amendments framed that way read as a firm doing its administration properly rather than a firm being difficult, and buyers accept them far more often than subcontractors expect. What signature changes is the price of the conversation: before it you are trading terms, after it you are asking for favours.

Picture a drylining subcontractor sent an order with the final date stretched to 60 days from the Due Date and a clause voiding any application submitted late. It replies with two amendments: the final date at 30 days, and late applications carried to the next cycle instead of lost. The buyer accepts the second immediately, comes back at 45 days on the first, and the subcontractor takes the compromise having priced the extra fortnight of financing into its tender. Nothing about the exchange soured the relationship; the buyer had simply opened with its best case. The lesson is that amended payment terms are an opening position, and the week before signature is when they are cheapest to move.