The 37-year-old, who is currently a free agent after ending his 19-year stay at Juventus in May, had appeared to be on the verge of joining the Australian side last week before his agent revealed that Olympiakos were also in talks with him.
"We are optimistic, but Del Piero hasn't given definitive assent and he hasn't signed yet," Pignata told reporters in Italy.
"Talks are still open. There are a few aspects to define. But Australia is ready to wait for Del Piero."
Del Piero was capped 91 times at senior international level, scoring 27 goals.