(Reuters) -Australia's ASX said it will cancel all trades in TPG Telecom shares in the first 15 minutes of Wednesday's session, after a mix-up by the exchange operator caused the stock to fall about 5% before trading was halted.

The ASX incorrectly tagged TPG Telecom in an announcement by software firm Infomedia agreeing to a A$651 million ($422.2 million) takeover by private equity firm TPG Capital, an entity unrelated to Australia's third-largest telecom operator.

ASX said the announcement was "erroneously cross-released by ASX against TPG Telecom", and that it would cancel all trades in TPG Telecom's securities before the halt went into effect at 10:15 a.m. in Sydney.

"This mistake shouldn't have happened, and we are reviewing our internal processes to understand if there are additional safeguards or procedures we could implement to reduce the risk of a similar reoccurrence," said Darren Yip, ASX's group executive markets and listings.

"While we welcome the ASX's decision to cancel trades made during this window, we expect a full explanation of how the error occurred and what steps will be taken to prevent similar incidents in future," a TPG Telecom spokesperson told Reuters in an emailed statement.

Trading in TPG Telecom's stock resumed just after midday local time, and was last trading 1.6% lower at A$5.410 per share. The early trades wiped off more than A$400 million from TPG Telecom's market value.

($1 = 1.5418 Australian dollars)

(Reporting by Sameer Manekar in Bengaluru; Editing by Mrigank Dhaniwala and Sonia Cheema)