Greg Combet felt "entitled to trust" ex-CFMEU boss John Maitland after years spent side by side in the union movement.

But it turned out there were certain things his old colleague was keeping from him, Mr Combet told a corruption inquiry on Friday.

The Federal Climate Change Minister and former ACTU secretary was grilled at the Independent Commission Against Corruption (ICAC) in Sydney over a 2008 letter he wrote, urging then-NSW minister Ian Macdonald to support a training facility in the Hunter Valley.

Mr Combet told the commission he supported the idea of a training mine at Doyles Creek.

But he testified he did not realise Mr Maitland was seeking an exploration licence that could allow the establishment of a large commercial mine from which Mr Maitland stood to net millions.