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It takes the Forrests' total contribution to $2 billion since 2001.
The Forrests said their latest donation was based on their belief that "we all have to look out for each other" had never been stronger.
"In just the last six months we've seen the devastating impacts of bushfires on many of our fellow Australians and now the global impacts of COVID-19, a threat unprecedented in our lifetimes," Andrew Forrest said.
"It is incumbent upon us to help where we can. This additional funding will provide both the capacity and the flexibility to allow the foundation to continue its outstanding work and tackle new challenges as they arise for many years to come.
"At a time when a lot of people both here and around the world are hurting, we are more determined than ever to step up and take on the most pressing global challenges."
The additional funding will go towards the COVID-19 response, the Minderoo Foundation Fire Fund assisting with bushfire relief, and ‘Sea the Future', an industry-focused initiative to end plastic waste worldwide.
Minderoo's other causes include cancer research, childhood development, ocean health and marine life, human rights, the arts, technology, education and indigenous causes.
"We continue to encourage other high-net-wealth individuals to commit their leadership, energy and wealth to the causes they are passionate about," Nicola Forrest said.
"The generosity we have seen from individuals, corporations and philanthropic organisations in responding to Australia's recent bushfire crisis and now COVID-19 has been heart-warming.
"We truly believe the generosity of the human spirit is alive and well and is critical not only to see us through times of crisis but also to seek out disadvantage at all times and eradicate it."