The Transport Workers Union forced its members to join the industry superannuation fund which paid its directors, including senior union officials, $200,000 in fees each year, the royal commission into trade unions has heard.

The royal commission was told this conduct raised a potential conflict of interest.

The hearing in Perth on Monday was told that it was a requirement of enterprise bargaining agreements struck in 2011 and 2013 that TWU members join the fund.

Jeremy Stoljar, counsel assisting the royal commission into trade union governance and corruption, said four of the nine directors of TWU Super were among the most senior officials of the union.

The head of the negotiating team for the TWU enterprise agreements of 2011 and 2013 was Michael Kaine, an alternate director for the board of TWU Super.