BEIJING, May 18 (Reuters) - The National Development and Reform Commission (NDRC), China’s state planner, said at a news conference on Tuesday that Australia should treat Chinese firms fairly and stop interfering with trade and investment between the two countries. Beijing earlier in May “indefinitely” suspended all activity under a China-Australia Strategic Economic Dialogue, amid increasingly strained relations between the two countries. The NDRC said on Tuesday that Australia should take full responsibility for this suspension. (Reporting by Stella Qiu and Gabriel Crossley; Editing by Kim Coghill) Our Standards: The Thomson Reuters Trust Principles.