The Australian Rugby Union played a big part in securing the signatures of Israel Folau and Quade Cooper, boosting their salaries with secret top-up deals.

The Sydney Morning Herald reports Folau will be paid $400,000 by the ARU on top of the $250,000 he receives for his one-year deal with Super Rugby club the Waratahs.

Folau's agent Isaac Moses denied the serial code-hopper was receiving any extra payments, while ARU acting CEO Matt Carroll also declined to comment on any top-up deals.

''We never have and never will divulge what we pay," Carroll told the Herald.

Former NRL star Folau, who walked away from AFL club GWS Giants after just two years, will earn $650,000 for his one season in rugby union – his third football code in four years.

''The ARU have definitely assisted us but don't declare the amount with us. It's their choice to talk about salaries,'' Jason Allen, Waratahs chief executive, said.

Outspoken Wallabies star Cooper is also believed to have received an ARU top-up on his Queensland Reds contract.

The Reds announced a $400,000 extension of Cooper's contract on Thursday with the ARU putting a further $350,000 on top of that figure.

The amounts of the Folau and Cooper contract top-ups from the ARU are similar to sums paid to rugby league defectors Wendell Sailor, Lote Tuqiri and Mat Rogers when they moved to rugby union.

Cooper's agent, Khoder Nasser, and Moses are not accredited with either the ARU or NRL.