Australian heavyweight boxer Solomon Haumono has labelled Sonny Bill Williams "a soft target" and challenged former world champion Danny Green to fight him instead.

Green, a four-time world champion in the super-middleweight, light-heavyweight and cruiserweight divisions, has bizarrely sought to fight Williams next year despite the rugby and league superstar not having been in a boxing ring since January when he won his seventh fight.

That was noted by Haumono who released a statement calling out Green for a fight to liven up the Australian scene.

Green retired in 2012 after beating New Zealander Shane Cameron but returned to action this year, beating Bolonti.

Now Williams, who hasn't responded to the temptaiton of taking on Green, has found himself caught up in war of words across the Tasman.

"Why pick Sonny when he is only a part-time boxer who hasn't fought in nearly a year? Danny is world class as we saw in his last fight when he easily beat an opponent who'd just fought for a world title," Haumono said as Sydney's Daily Telegraph picked up on the challenge.

"Green against SBW is a ridiculous mismatch. Danny is a former world champion who has boxed over 200 rounds as a professional. Sonny's only had seven bouts and 34 rounds.

"Danny has a history of picking soft targets, like Paul Briggs who hadn't fought in nearly three years after retiring on medical grounds.

"Then he targets Choc [Anthony Mundine] who is coming off a bad loss and has been fighting four weight divisions below him at junior middleweight.

"My message to Danny Green is loud and clear – if you want to move up with the big boys at heavyweight – then look no further. I'm willing to face you anywhere, anytime.

 "I am world-rated, and like Choc and Sonny Bill I have played rugby league at the highest level.

"Let's dance Green Machine. My manager Matt Rose will welcome your call anytime, if you find the courage."

Auckland-born Haumono played for five clubs in a 10-year NRL career but dabbled in boxing from 2000 and has concentrated on that since 2004, building a professional record of 23-2-2 (20 KOs). he is ranked No 14 in the WBA.

Double Rugby World Cup winner Williams has expressed a desire to continue to box. But the multi-talented athlete has a new challenge on his hands in the form of rugby sevens as he chases an Olympics dream.