The Australian Federal Police has been asked to investigate Australian engineering company UGL's secret multimillion-dollar payments to Hong Kong Chief Executive C. Y. Leung.

Greens leader Christine Milne on Thursday referred the matter to newly installed federal police commissioner Andrew Colvin for investigation of potential breaches of Australia's foreign bribery laws.

Senator Milne's referral came after Fairfax Media published documents detailing a secret $7million fee arrangement between Mr Leung and the ASX-listed UGL.

Hong Kong's feared anti-corruption watchdog has also been urged to investigate the deal, and opposition Hong Kong democrat lawmakers have threatened moves to impeach Mr Leung if he does not sufficiently explain why the payments remained undisclosed.

Democratic Party lawmaker Albert Ho called on Hong Kong's anti-graft agency the Independent Commission Against Corruption to investigate, saying the deal, first revealed by Fairfax Media, cast a "very bad light on the integrity" of Mr Leung.

"As legislators we owe a duty to find the truth to enable the public to know what the Chief Executive has done, whether or not he is still suitable to continue to be our Chief Executive," Mr Ho told reporters in Hong Kong's Legislative Council offices. "Very probably there will be impeachment proceedings to follow."

But, Mr Ho said, the Chief Executive would first be given the right of reply, adding that his party had already planned to impeach Mr Leung due to his handling of the city's unprecedented pro-democracy protests.

Late on Thursday, Hong Kong called off talks with protesting students, dealing a heavy blow to attempts to diffuse a political crisis that has seen tens of thousands take to the streets, calling for free elections and demanding Leung resign.

Senator Milne's letter to Commissioner Colvin includes a copy of a December 2011 letter and schedule of agreement written by UGL's chief executive Richard Leupen and signed by Mr Leung.

She wrote that the letter "may assist the Australian Federal Police determine whether any Australian laws relating to corporate crime or bribery laws have been breached".

Under Australia's foreign bribery laws, it is illegal to provide a benefit to a foreign public official in order obtain a business advantage.

Both UGL and Mr Leung have denied any wrongdoing.

UGL and Mr Leung's office have stated that neither party was obliged to disclose the arrangement because Mr Leung was not an elected official at the time it was signed. Mr Leung's office said the payments related to past, not future, service.

The fee agreement between UGL and Mr Leung was signed months before he was elected to Hong Kong's most senior political office in March 2012.

Mr Leung had just announced his candidacy when the agreement was signed on December 2, 2011.

However, UGL paid Mr Leung in two instalments, in 2012 and 2013, after he assumed the office of Hong Kong Chief Executive.

UGL has said the agreement was necessary for it complete a smooth acquisition of insolvent British-listed property services company DTZ Holdings, which took place two days later.

Mr Leung held a considerable shareholding in and sat on the board of  DTZ at the time of its demise. He also ran the company's Asian operations.

His shares became worthless when it entered administration on December 4,  2011.

A Hong Kong source familiar with the details of the deal between Mr Leung and UGL told Fairfax Media the politician was upset by his losses and believed the payments from the Australian company would help "top up" his finances.

A former senior anti-corruption investigator told Fairfax Media Mr Leung now faced the most serious questions of his political career.

Lam Cheuk-Ting, who stepped down from Hong Kong's ICAC  in 2011 to become chief executive of the city's Democratic Party, said the secret contract raised questions for the watchdog to examine.

"I urge ICAC to carry out an independent investigation without fear, without favour, of these very, very serious allegations," Mr Lam told Fairfax.

Mr Lam said the integrity of ICAC and Hong Kong's broader legal system was at stake following a series of appointments of people who were close to Mr Leung and his backers in Beijing.

Global media outlets and independent sections of the Hong Kong media have been critical of Mr Leung's initial response to  Fairfax Media's first report on Wednesday.

David Webb, a corporate governance expert, told the Financial Times that Mr Leung's $7million was "more money than 90 per cent of Hong Kong citizens will earn in their lives".

The Apple Daily published an animated news video showing Mr Leung signing contracts under a table with a kangaroo, before a rival uses a crowbar to hoist him out of his Chief Executive's chair.

UGL on Thursday  released a statement to the Australian Securities Exchange describing the fee arrangement as a standard business practice to ensure Mr Leung did not compete against DTZ or poach key staff.

"Such agreements are common confidential commercial arrangements when a business is being acquired. The only difference here being Mr Leung went on nearly six months later to become the Chief Executive of Hong Kong," UGL's statement said.

"UGL was under no obligation, legal or otherwise, to disclose the agreement. It should be noted that at the time of entering in the agreement, Mr Leung was not an elected official of Hong Kong, and UGL had no reason to expect that his campaign for Chief Executive of Hong Kong would be successful. In any event the same commercial protections for UGL and DTZ were necessary."

Mr Leung's spokesman Michael Yu said earlier this week: "The payments therefore arise from Mr Leung's resignation from DTZ, not any future service to be provided by him."