Former senior Labor ministers Michael Costa, Tony Kelly and Joe Tripodi are to be embroiled in a fresh investigation by the Independent Commission Against Corruption which has launched another fully-fledged graft inquiry into the former minister Eddie Obeid.

The Herald can reveal that the corruption watchdog has held secret compulsory interrogations and examined thousands of documents in an attempt to further unravel Mr Obeid's business dealings with a state government he once controlled from the shadows.

Senior public servants and others have confirmed that ICAC investigators are focusing on three lucrative government leases the Obeids controlled for many years at Circular Quay and on a secret interest negotiated by the Obeid family in Australian Water Holdings, a project management firm that manages hundreds of millions of dollars worth of work for Sydney Water.

The ICAC has seized volumes of material from government agencies and has been interrogating bureaucrats over whether they were pressured to make decisions advantageous to Mr Obeid's interests.

Apart from former treasurer Mr Costa, former planning minister Mr Kelly, and former waterways minister Mr Tripodi, senior Liberal Party figures, including federal senator Arthur Sinodinos and backroom fund-raiser Nick Di Girolamo, are expected to be in the spotlight given their former senior roles in AWH.

On Wednesday, the ICAC handed down landmark findings that branded Mr Obeid and his son Moses Obeid as corrupt, having rigged a major coal tender in 2008 with the assistance of the corrupt former mining minister Ian Macdonald. The Obeids made $30 million from this coal deal and stood to make a further $100 million.

But the family stood to gain a similar windfall from a plot to have part of Sydney Water privatised and sold to Australian Water Holdings, a company formally run by Mr Di Girolamo. This was to have been a billion-dollar deal.

After a Herald investigation into the affair last December, Mr Di Girolamo issued several legal threats against the newspaper. In April, after damning evidence of his secret negotiations with the Obeids, Mr Di Girolamo suddenly departed the company, selling down his 62 million shares. He did not return the Herald's calls on Thursday.

Another sudden departure from AWH was Greg Skehan, a Colin Biggers & Paisley solicitor. Mr Skehan was revealed to have acted as a director of a company used to hide the Obeids' interest in one of the coal deals examined by ICAC.

Investigators have been piecing together a secret investment in the company that the Obeid family negotiated with Mr Di Girolamo, who is a close friend of Eddie Obeid jnr.

In December last year, Mr Di Girolamo denied in an interview with the Herald that any of the Obeids had ever had any interest in AWH: ''I have never been involved in a transaction with the Obeids. I have never had anything to do with the Obeids.'' He went on to admit that he had discussed the company with Eddie Obeid snr, had employed Eddie Obeid jnr for up to 18 months in the company, and that at the end of 2010 had received a personal loan from Eddie jnr for a sum of money he refused to disclose. During the recent hearings, a forensic accountant traced $3 million of the proceeds from the Obeids' corrupt coal deal into AWH.

A well-placed source said ICAC had demonstrated a particular interest in the involvement in the affair of former lands and planning minister Mr Kelly. His predecessor, Frank Sartor, in a book published after he left office, alleged that a cabinet minute dealing with the proposed AWH/Sydney Water deal was radically changed after it was sent to Mr Kelly's office.

''When the draft minute was sent to planning minister Kelly's office it was completely redrafted to support the PPP. Alarmed bureaucrats from relevant agencies drew the matter to the attention of the Premier's office and the minute never made it to cabinet,'' he wrote.

''I don't want to comment and I can't,'' Mr Kelly said on Thursday.

Mr Costa put an end to a tender already advertised by Sydney Water that would have threatened AWH's future contracts with the state government. Three years later, in November 2011, Mr Costa was appointed chairman of AWH and given 5 per cent of the company. Mr Costa declined to comment on Thursday, but previously told the Herald it was ''absurd'' to suggest there was any connection between these events. The Herald does not suggest otherwise.

Mr Costa's appointment came about after a meeting with Eddie Obeid jnr and Eddie Obeid snr, who urged him to take the position.

Mr Costa replaced Liberal Party heavyweight Arthur Sinodinos, who was previously the company's chairman. Senator Sinodinos told Federal Parliament in March that he was ''shocked and disappointed'' to discover AWH was ''financially linked to the Obeid family''.

He also abandoned his 5 per cent shareholding in AWH, which had been held by Mr Di Girolamo in a a ''gentleman's agreement''.

Another shareholder in the company is Joseph Georges, a Strathfield real estate agent who elsewhere has acted as a front for the Obeids.

Also of interest to ICAC are three Circular Quay leases which the Obeids controlled through another frontman, John Abood, who is Eddie Obeid's brother-in-law.

It is understood Mr Obeid lobbied several ministers over the leases. In 2009, NSW Maritime issued new 10-year leases to the incumbents along the wharves - including to the Obeids' front company - and again without a competitive tender. The minister at the time was Mr Tripodi, and the head of Maritime was Steve Dunn, Mr Obeid's chief bureaucrat when he was fisheries minister.

Mr Obeid's diaries show he met privately Mr Dunn no fewer than 11 times. Records obtained by the Herald demonstrate Mr Dunn showing a keen interest in the Obeid leases at about the same time as these meetings.

On Friday, December 7, 2007, for example, they met at 10.30am at the Sydney Hospital cafe. Five days later, Mr Dunn, then the general manager of the Maritime property division, emailed another Maritime employee to ask for Mr Abood's telephone number.

Government sources say ICAC has been making inquiries into a sudden change in policy under Mr Dunn to allow existing lessees to renegotiate renewals of their leases.

''The draft [policy] initially required all sites to be offered to the open market, but by the stroke of a pen at some point during the Steve Dunn reign another element was added … where direct negotiations over leases can be entered into with sitting lessees,'' a government source said.

Mr Dunn denied having done any favours for Mr Obeid, but admitted holding several meetings with the former ALP kingpin.

''Those meetings were to deal with certain issues,'' he said. ''I'm not happy talking about my employment at NSW Maritime because they are matters that related to my employment and they concern matters that may be commercial in confidence.

''I know there are reasons now to be very circumspect about meeting with Mr Obeid. Certainly they were not apparent at the time. All my dealings with Mr Obeid were above board - he never asked me to do anything inappropriate.

''Mr Obeid was a senior politician. Forget the fact he was not a minister any more. He was one of the most senior politicians in the NSW Labor government.

''Everybody had at some time cause to have some interaction with him who was in a senior position.''

Investigators Turn Attention To Three Former Ministers