Three directors of a Great Barrier Reef charity entrusted with almost half a billion dollars in public money have refused to give evidence to a Senate inquiry scrutinising the controversial deal, raising the prospect they will be forced to appear.

Confidential Senate committee documents seen by Fairfax Media show that despite being offered five dates at which to attend the inquiry, the directors of the Great Barrier Reef Foundation say they are unavailable for questioning, variously citing overseas travel commitments, medical appointments, board meetings and other unspecified engagements.

The inquiry was launched following the Turnbull government’s decision to grant the small, business-focused charity $443 million to help rescue the reef. The foundation has previously said it would “fully co-operate” with the probe.

The contentious Great Barrier Reef Foundation grant is to be spent on projects such as water quality improvements.

The Senate committee had specifically requested their attendance. The trio comprises the organisation’s chair John Schubert and board members Grant King and Paul Greenfield. Mr King is president of the Business Council of Australia and Dr Greenfield chairs the foundation’s scientific committee.