A "centrally relevant" witness in a multimillion-dollar cartel conduct case against Moses and Paul Obeid and Sydney businessmen John McGuigan and Richard Poole has vanished overseas, the Federal Court has heard.

The hearing of the cartel conduct case, launched by the Australian Competition and Consumer Commission last year, kicked off on Monday with allegations the Obeids received "substantial benefits" including $28 million in cash for rigging a NSW government tender for two lucrative coal exploration licences.

Moses Obeid and and Paul Obeid, right, leave the ICAC in April 2014. Credit:AAP

One of the licences was over Cherrydale Park, the Obeid family's farm at Mount Penny in the Bylong Valley.

The court heard an alleged Obeid frontman, Andrew Kaidbay, left the country last year and "attempts to locate him haven't been successful".