FFA is currently determining how many of the extra seats could be set up for the match but there are "operational inefficiencies", as A-League chief Greg O'Rourke described them, which must be accounted for and may mean not all of them are used. Those issues include the blocking of LED signage as, the potential to obstruct the views of spectators positioned in the lower part of the permanent stands, and if the exercise is financially worthwhile.

"My guys are working through all those options ... if we look like we're going to blow past 60,000, then they're on standby," O'Rourke told the Herald. "I think we're in with a chance of knocking off the A-League record. After that, who knows?"

The biggest crowd to have attended a sporting fixture at Optus Stadium is 59,608 for the West Coast Eagles' AFL preliminary final against Melbourne in September last year.

In the A-League, the largest grand final crowd came in the 2006-07 decider when Melbourne Victory smashed Adelaide United 6-0 in front of 55,436 fans at Marvel Stadium. That's still short of the all-time benchmark of 61,880 for the Sydney derby at ANZ Stadium in October 2016.

One or all of those records could be under threat on Sunday, but FFA executives are privately talking down the chances of a full sell-out in the expectation that the rate of sales will slow in the coming days. Many of the remaining tickets are in higher-priced sections and some are individual seats, making them potentially less attractive to buyers.