The dealings between former state Liberal energy minister Chris Hartcher and the head of a company linked to the Obeid family are set to be exposed in a new corruption inquiry into political donations that begins public hearings this month.

The Independent Commission Against Corruption is probing allegations that former Australian Water Holdings (AWH) chief executive and Obeid associate Nick Di Girolamo arranged for the company to make “regular payments” to Eightbyfive – an alleged Liberal Party slush fund set up by Mr Hartcher’s former adviser Tim Koelma – in exchange for favourable treatment from the former minister.

An inquiry into AWH, which concluded public hearings on Wednesday, heard allegations the company paid Eightbyfive more than $7000 a month, purportedly for “media, public relations and other services” from Mr Koelma.

Mr Di Girolamo, whose gift of wine triggered the resignation of premier Barry O’Farrell, admitted this week he was “close to Chris Hartcher” and “close to his office”.

Mr Koelma has been implicated in an attempt to smear former Sydney Water boss Kerry Schott by making a false corruption complaint.

The previous inquiry heard corrupt former Labor minister Eddie Obeid predicted a complaint would be made against Dr Schott, a respected bureaucrat, who opposed AWH’s plans for a public-private partnership with the government.

“Eddie Obeid was right on the money,” counsel assisting the commission, Geoffrey Watson, SC, said earlier this year. “ICAC . . . has been able to track down the source of the complaint. This is where the misconduct makes its leap across party lines.”

The inquiry into political donations, dubbed Operation Spicer, is due to start public hearings on April 28. It is expected to reveal the source of the corruption complaint and air allegations of favourable treatment secured by AWH from Mr Hartcher. Mr Hartcher quit cabinet last year after ICAC investigators raided his central coast office.

The commission has heard Operation Spicer is also concerned more generally with “political fund-raising in the Liberal Party, and the way in which unscrupulous businessmen sought to buy political influence”.

The inquiry will examine allegations Mr Hartcher, fellow central coast Liberal MPs Chris Spence and Darren Webber, Mr Koelma and Raymond Carter, “corruptly solicited, received, and concealed payments from various sources in return for certain members of Parliament favouring the interests of those responsible for the payments”.

ICAC spotlight turns to Hartcher