SYDNEY (Reuters) -Australia's corporate regulator said it had launched a broad investigation into the country's main stock exchange operator, escalating tensions that have simmered for years amid a botched software upgrade and a series of trade-processing glitches.

The Australian Securities and Investments Commission (ASIC) said on Monday that it and the Reserve Bank of Australia (RBA), the exchange's joint regulators, held ongoing concerns about the ability of ASX to maintain stable, secure and resilient critical market infrastructure.

ASIC was already reviewing a December 2024 malfunction of ASX's settlement technology. But the regulator said it would cancel that project and order an expert panel to investigate the "governance, capability and risk management frameworks and practices across the group".

Shares of ASX were down 7% in afternoon trading, their biggest intraday fall in a year, against a flat overall market, as the company's relationship with regulators deteriorated further after years of rebukes.

ASIC Chair Joe Longo did not specify what prompted the ratcheting up of scrutiny except to say in a statement that "the decision to initiate an inquiry follows repeated and serious failures at ASX".

The regulator would publish the investigation's findings, although the timing was still to be decided, ASIC added.

ASX said it acknowledged the seriousness and significance of ASIC's investigation and that it was cooperating with the regulator.

"We have been working hard on a transformation strategy ... but we acknowledge there have been incidents that have damaged trust in ASX," ASX Chairman David Clarke said.

The RBA declined to comment.

ASX, which processes most of Australia's stock trades and runs all of its clearing and settlement services, experienced the first of a series of technical glitches that resulted in a near full-day outage in the heightened trading volume of 2020, sparking an ASIC probe at the time.

ASX meanwhile frustrated market participants - and ultimately ASIC and RBA - with an ambitious, ultimately disastrous, attempt to rebuild its clearing and settlement software platform with custom blockchain-like technology from 2017.

After delays, expenses and mandatory industry consultations, ASX abandoned the project in 2022, and the following year it hired India's Tata Consultancy Services to start afresh on a staged upgrade. The first part is due to be delivered in 2026, ASX has said.

(Reporting by Byron Kaye in Sydney and Rajasik Mukherjee in Bengaluru; Editing by Sonali Paul, Sherry Jacob-Phillips and Jamie Freed)

By Byron Kaye