The court heard how Haiu went to Cooper's property in May and punched the property developer twice in the face.
He pleaded guilty to common assault and possession of an offensive weapon, a hammer.
Police withdrew a third charge of assaulting Cooper's partner, Kirsten St George.
Judge Gerard Winter ruled that Haiu's criminal record would remain clean so he could continue his battle with cancer, work overseas and support his wife and two children.
He said the Blues player was a role model to many, the attack was provoked by Cooper's actions and Haiu had accepted responsibility for his assault.
Haiu's lawyer, Richard Earwaker, earlier told the court there were several reasons behind the attack. Haiu had lost $400,000 to $500,000 to Cooper after investing in unsuccessful property deals.
The rugby star has played 72 games for Auckland and 53 games for the Blues but has been on indefinite leave from rugby after being diagnosed with Ewings sarcoma, an unusual tumour, on a rib.
The cancer diagnosis caused Haiu to lose a contract to play rugby in Japan, putting further financial pressure on his family.
