The salaries of the executives at the body overseeing the WestConnex motorway project appear higher than those of other top NSW public servants but their actual size will remain secret because Sydney Motorway Corporation refuses to reveal how many senior managers are on its books.

The latest annual report for Sydney Motorway Corporation shows the amount paid to senior executives in the 12 months to June totalled $5.15 million, up from $1.21 million a year earlier.

The report lodged with the corporate regulator lists SMC's four executives during the 2015-16 financial year as chief executive Dennis Cliche, chief financial officer John Kite and two others who did not serve a full 12 months. Executive director Peter Regan resigned in November 2015, while Roderick MacKinnon was appointed company secretary in February 2016.

SMC has declined to reveal the total number of "key management personnel" during the year, which means taxpayers will remain in the dark about how much each of its senior executives were actually paid.