CSIRO chief Larry Marshall has ruled out any discrimination against staff on the basis of union affiliation or location despite internal emails showing such factors might play a role during the agency's painful job cuts.

Private emails released to a Senate inquiry last week included one noting a staff member whose age was "over 60" and with "strong union links". Another email has a CSIRO official pledging to work to "minimise impacts at Hobart" because of the need to address employment losses in Tasmania, "a regional political issue".

Dr Marshall was questioned over the issue at the latest Senate committee hearing on Thursday in Canberra examining the process and rationale behind CSIRO's plans to lop 350 jobs, including many climate scientists, and to rehire any staff in other growth areas over the next two years. The staff union says job losses may reach 450.

The emails were "surprising", Dr Marshall told the senators, including shadow industry minister Kim Carr (ALP), who noted such discrimination was contrary to CSIRO codes of conduct and probably illegal.

"[Am I] ruling it out? Absolutely, they are not part of our decision criteria, Senator," Dr Marshall said.

The two-hour-plus grilling, though, failed to generate any major new insights into the logic of the cuts, which have drawn protest from scientific bodies at home and thousands of scientists abroad.

Dr Marshall did disclose that he had had a salmon barbecue at the home of then communications minister Malcolm Turnbull in early 2015, not long after the former venture capitalist returned to Australia from the United States to take on the CSIRO job on a two-year contract with a three-year extension option.

Dr Marshall refused to say whether he had discussed CSIRO matters with Mr Turnbull, prompting Liberal Senator Sean Edwards to dub the occasion "Salmongate".

"Again, I don't think it appropriate to discuss the content of a conversation with a federal minister," Dr Marshall said, when pressed to reveal what he discussed with the now Prime Minister.

Before Thursday's meeting, CSIRO disclosed to the Senate hearing that senior staff at a "team planning meeting" on  November 28 last year had decided to start using private email communications, although "no specific instruction was given".

"This approach was for the purpose of ensuring confidentiality and avoiding undue stress to other staff," a statement from Hazel Bennett, CSIRO's chief financial officer, said.

Ms Bennett told the inquiry that 17 staff had resorted to private email use, according to a probe by CSIRO. Dr Marshall was not among those examined.

Greens and Labor senators had been hoping for the release of more private emails and plan to hold further hearings with Dr Marshall and also the CSIRO board.

"There's so much we just don't know," Janet Rice, Greens senator from Victoria, said at a media briefing. "There's the whole period of time from mid-December to mid-January.

"Larry Marshall was completely evasive over who he consulted during that period of time to determine we were going to have these massive cuts to climate science," Senator Rice said.

Fairfax Media sought comment from CSIRO about when other private emails would be disclosed.