However the company is still maintaining its profit guidance for the current financial year.
Chief executive Andrew Roberts said the risk of a further cost overruns in the A$1.2billion reconstruction of the home of English football had worsened.
He suggested a worst-case scenario would be for about £25million ($A60 million) to be wiped off profits due to the cost of its steel contractor, Hollandia, at the stadium.
Multiplex is now in negotiations with Hollandia to set an upper limit on the remaining costs, with the contractor putting forward the £25million figure.
In June, the company was forced to admit a A$109 million loss on the project.
Roberts confirmed that the project would win the race to be ready for the FA Cup final next May.
