Prime Minister Tony Abbott has declared confidence in Assistant Treasurer Arthur Sinodinos, who has been drawn into a NSW anti-corruption inquiry.

The NSW Independent Commission Against Corruption has opened hearings in regard to Australian Water Holdings (AWH), a company which Senator Sinodinos joined as a director in 2008 before he entered parliament.

It is alleged the family of disgraced former Labor powerbroker Eddie Obeid bought a 30 per cent stake in AWH, after which Mr Obeid lobbied fellow NSW Labor ministers to approve a multi-million-dollar business contract between state-owned Sydney Water and AWH.

The contract would have benefited now Senator Sinodinos by up to $20 million in bonuses, the inquiry has heard.

The minister - who will appear as a witness at the inquiry in coming weeks - told parliament in February 2013 he had not been aware of Mr Obeid's involvement or that of the Obeid family when he joined the company.

He also said in that speech that he had foregone his entitlement to a shareholding in the company.

ICAC is also looking at whether the former NSW Liberal Party executive member's placement on the company board was aimed at opening lines of communication with the Liberals.

The inquiry has heard AWH charged administration costs to Sydney Water, including $75,636 in donations to the Liberal Party - which the party has decided to refund.

Mr Abbott told parliament the former chief of staff to John Howard had served the country "long and faithfully" and he stood by the minister.

"People should be in public life to serve the country, not themselves," he said.

Labor grilled the minister about the ICAC inquiry in the Senate on Tuesday.

Senator Sinodinos said he would be appearing at the corruption inquiry as a witness but stood by his earlier statements professing his innocence.

"Watch this space, I will be vindicated in terms of what I've said to the Senate," he said.

Labor senate leader Penny Wong said the minister's earlier statement on the matter was "inadequate at best and deliberately obfuscatory at worst".

Labor MP Kelvin Thomson called for the minister to step aside from the frontbench or be sacked.

"I think it would be in the best interests the integrity of the system if Senator Sinodinos were to step aside," he said.