Hochtief announced a $1.16 billion share buying offer on Monday to acquire three out of every eight Leighton shares held at a price of $22.15 cash per share – an 18.8% premium over the dividend-adjusted five-day volume weighted average price of Leighton shares.

The offer sent the share price up 11.4%.

But there are concerns about insider trading, as Leighton shares had already spiked 14% on Thursday and Friday before Hochtief’s bid was announced on Monday, suggesting some traders held price-sensitive information.

On Monday, the Australian Securities Exchange wrote to Leighton asking when the company first became aware of Hochtief’s offer and whether it disclosed the offer prior to the announcement.

“The chairman of the company, Mr Robert Humphris OAM, was informed on Sunday, March 9, 2014 at approximately 12.30pm, at a meeting with Mr Marcelino Fernandez Verdes, chief executive officer of Hochtief AG, that Hochtief was proposing to make a proportional offer,” Leighton wrote back to the ASX.

“The terms of the offer were not provided at this meeting.”

The contractor explained it received a formal offer in the morning of the next day, which is when the shares were put in pre-open from 10am with the understanding that the legal advisors at Hochtief Australia Holdings would provide the Hochtief announcement to the ASX.

The announcement was released at 10.21am that same morning. An announcement by Leighton followed at 10.37am.

“At no time did the company’s securities trade when information regarding the proposed offer was in company’s possession but not released to the market,” Leighton further wrote.

“The company confirms it is in compliance with the Listing Rules and, in particular, Listing Rule 3.1.”

Still, ASIC said it would review Leighton share transactions made ahead of the announcement.

“We monitor the market in real-time so you would expect we would review the trading ahead of major corporate news,” ASIC spokesman Andre Khoury told the Herald Sun.

However, he said no formal investigation was underway.

Leighton noted the potential bid and said its independent directors recognised it was conditional and would only make a recommendation once a bidder’s statement was received, expected by the end of the month.

Germany’s Hochtief is controlled by Spanish construction giant ACS.

It holds 58.7% of Leighton Holdings but if its offer is successful, Hochtief’s stake will increase to 74.2%.

Hochtief said it intended to increase its representation on the Leighton board, though it continued to support Humphris.

Out of 10 board members, Hochtief holds three seats.