Former Labor factional kingpin Eddie Obeid and his family stood to increase their "massive benefit" by tens of millions of dollars if a coal mining lease is granted at Mt Penny in the NSW Bylong Valley, a corruption inquiry has heard.

Mr Obeid's son Moses has already agreed the family could reap at least $75 million from decisions taken by former NSW Labor minister Ian Macdonald.

But this figure could escalate because the Obeids retain a 9.3 per cent share of Cascade coal as a result of Cascade defaulting on a second $30 million payment to them, the inquiry was told on Friday.

Counsel assisting the Independent Commission Against Corruption (ICAC), Geoffrey Watson SC, suggested to Moses Obeid that the Mt Penny holding, 150km west of Newcastle, was valued at $500 million when it was the subject of an exploration licence, but could be worth $1 billion if a mining licence is granted.

"Possibly," Mr Obeid said.

The Obeids' share could jump from $50 million to $100 million, Mr Watson said.

"If you like," Mr Obeid replied.

Mr Watson suggested the "massive benefit" to the Obeid family that all started with information from Mr Macdonald could be $75 million or "a bit more ... $175 million".

Mr Obeid said the price of coal had fallen to half of the $160-a-tonne it was fetching when the previous valuation was made.

But when Mr Watson asked whether a mining lease would make their holding much more valuable, Mr Obeid replied: "I'm not disagreeing with you. I hope you are right."

Mr Obeid said his 69-year-old father was not actively involved in the family business, and was happy for his sons to run and control it.

"At his age he's not interested in the mechanics of particular deals," he said.

The ICAC is investigating claims that Mr Macdonald rigged a 2008 tender process for coal exploration licences in the Bylong Valley.

It is also investigating whether Eddie Obeid and his family gained substantial financial benefit from it.

Moses Obeid was unable to explain in earlier evidence on Friday how highly confidential government maps of the Mt Penny coal tenement came to be in the family's possession.

When Mr Watson suggested that pencil marks on one map, reflecting the redrawn boundaries of the tenement on land the Obeids later purchased, might have been made by Mr Obeid after a conversation with Mr Macdonald, Mr Obeid replied:

"They might have been drawn by Jesus Christ."

Mr Obeid faced further accusations of telling lies. At one point Commissioner David Ipp asked whether he was prepared to lie only to protect the family's privacy in business, or whether he would lie to shield his father's involvement in business dealings. Mr Obeid said he would not.

Eddie Obeid's wife Judith gave evidence, but despite being a director of the family company she said she knew nothing of its financial transactions, left everything up to her sons and her memory had been affected by surgery to remove a brain tumour.

Mr Obeid Snr is expected to testify on Monday.