The Forrests were among the first international philanthropists to sign up to the Giving Pledge, a US initiative spearheaded by billionaires Warren Buffett and Bill and Melinda Gates aimed at increasing charitable giving among the wealthy.

Other mining magnates to pledge included African Rainbow Minerals founder Patrice Motsepe and his wife Precious from South Africa and Russia's Vladimir Potanin of Norilsk Nickel fame.

In his pledge letter, Forrest said giving back was more satisfying than the exhilaration of building a company.

He said giving time and money was demanding but he was guided by the principles of the New Testament when helping the less fortunate.

"In our country, this is our first Australians and globally, those suffering the unbearable yoke of modern slavery and forced labour," he wrote.

Forrest reiterated his opposition to cash handouts.

"Our aim is to significantly improve people's lives and happiness through their own independent sustainability," he said.

Forrest is well known for his philanthropy, stepping aside from his role as chief executive officer of FMG in 2011 to devote more time to his charity work.

He and his wife set up the Australian Children's Trust 11 years ago.

He is also behind the Australian Employment Covenant and Generation One initiatives to drive indigenous employment and has spoken out against modern slavery.

Forrest regularly gifts charities with FMG shares.

Forbes estimated Forrest's net worth to be $4.8 billion in January.