"We are rapidly reaching the point of no return for the planet," UN Secretary-General António Guterres said in a new video message.
He said the world faced a triple environmental emergency of biodiversity loss, climate disruption and escalating pollution.
"We are ravaging the very ecosystems that underpin our societies. And, in doing so, we risk depriving ourselves of the food, water and resources we need to survive."
The UN chief said the United Nations Decade on Ecosystem Restoration campaign was a chance to avert a climate catastrophe.
"Science tells us these next 10 years are our final chance to avert a climate catastrophe, turn back the deadly tide of pollution and end species loss."
'Koalas on death row'
Australians marked World Environment Day with tree planting, clean up events and conservation rallies across the country.
In Sydney's Hyde Park, around 200 people gathered to call for the NSW government to strengthen legal protections for koalas.
Conservationists say NSW koala populations have plummeted in the past 30 years. Before the Black Summer bushfires that devastated habitats, it was estimated to be fewer than 20,000.
A recent NSW parliamentary inquiry found that koalas in NSW could be extinct by 2050, because of the loss of habitat, disease and climate change events.
Nature Conservation Council NSW chief executive Chris Gambian warned koalas would join the Tasmanian Tiger as an extinct species unless there was a strict ban on the destruction of koala forests, and new nature reserves, habitat restoration and ecological research.
"Koalas in NSW are on death row," he said.
"Business as usual is simply no longer an option – this is an emergency that requires drastic action. Their numbers were plummeting before the Black Summer bushfires killed thousands of koalas and incinerated millions of hectares of forest.
"We will get to a point where our kids won't be able to show their kids koalas in the wild. You (would) only be able to see it in a museum or gallery."
There were also tree planting events across the country - including in western Sydney where a small group came together with the aim of planting more than 2,000 trees in the Penrith area.
Nicola Masters from Greening Australia said the initiative was important for the area, which suffers extreme heat events during summer.
"We're getting extreme urban heats and extreme weather conditions out here. The trees will act as a carbon sink, they will mitigate the effects of this increasing urban heat," she told SBS News.
Projects like this are what the United Nations wants more of.
But it says conservation efforts alone won't be enough to prevent widespread biodiversity loss and the collapse of ecosystems.
Hundreds of people also came together in Hobart's city hall, calling for protections for the Tarkine Forest in Tasmania’s north-west.
Former Greens leader Bob Brown, whose Bob Brown Foundation organised the rally, said the issue was galvanising people all over the country.
"We will stop this mad-brained destructive invasion of the Tarkine forest for a toxic waste dump that the company has got alternative sites for."