The severed link -- between Sydney and Melbourne -- meant that many traders had communication difficulties and couldn't make telephone calls to trade as normal.
An ASX spokeswoman said the ASX noticed that something adverse was happening this morning in the pre-opening phase between 9 and 10am. However, the market opened for trading as normal and switched to 'Adjust Phase' -- when nothing is trading -- at 10.19am.
The ASX claims the decision to cease trading was made to ensure the integrity of the market.
"The Optus network failure meant that some participants, particularly those in Melbourne, couldn't access the market. Trading was halted until all participants had resumed access," an ASX spokesman told ZDNet Australia. "To maintain the market's integrity is the fundamental objective of ASX," he added.
The ASX returned to 'pre-open' mode -- when traders can get in queue and enter buy and sell orders without actually trading -- at 11.40am and recommenced operations at 12.10pm.
The ASX refuted claims of an extended day's trade as a result of the communication breakdown, saying that this would only happen if there were massive volumes trading and investors couldn't get individual trades through before the market shut down.
"By 12.10pm it was like a normal market open with nothing particularly untoward happening," the spokesman told ZDNet Australia.
The ASX said it cannot recall a network failure of this kind in the past and expressed confidence in the market's ability to trade for the remainder of the day.
