The ACT Integrity Commission has been told of ministerial influence on contracts.

The watchdog this week heard that construction firm Manteena was informed it lost a Canberra school redevelopment contract due to differing ministerial approaches, despite outsourcing its competitor.

The ACT Integrity Commission is investigating the fairness of awarding a $18.8 million contract for Campbell Primary School's modernization to Lendlease instead of Manteena, which scored higher initially.

Manteena previously secured a contract to expand Canberra Hospital but faced labor relations issues.

Manteena CEO Mark Bauer has shared notes indicating ministerial influence on the contract decision.

Bauer says he felt the process was dragging and noted that differing ministerial approaches influenced the decision. An anonymous call also suggested a high-level departmental intervention.

The hearings continue.