Xenophon 'lacks substance': Bob Brown

Former Greens leader Bob Brown has questioned Nick Xenophon's environmental credentials while looking to shore up votes in South Australia.

Dr Brown says Senator Xenophon, whose party poses a strong threat to the survival of Greens Senator Robert Simms, is a "very clever politician" but lacks substance on key issues.

"I've never heard Nick say a thing about the Tarkine wilderness and you've got to listen hard to hear anything about the (Great) Barrier Reef. These are national issues," he told reporters in Adelaide on Monday.

Senator Xenophon said voters should remember the Greens' decision to preference self-professed climate skeptics ahead of his running mate Stirling Griff at the 2013 election.

"The personal attack by Bob is pretty sad," he told AAP.

"I think their credibility on this is damaged by some of the preference deals they've done in the past."