School children, teachers and parents gathered in Sydney holding placards that read "Time is running out", "Human change, not climate change", "There is no Planet B", and "If not now, when?", reports Efe news.
Thousands also gathered in other cities around the country, such as Canberra, Melbourne and Hobart as part of a global event aimed at raising awareness among governments and companies of the need to curb climate change.
On the streets of the Australian capital, demonstrators called for "immediate action" while taking a swipe at Prime Minister Scott Morrison, for his position in favour of the coal industry.
Morrison, who is in the US to meet President Donald Trump, will not attend the UN summit in New York on September 23, a gesture the Greens party described as "cowardice and contempt" given that Australia is one of the world's biggest greenhouse gas emitters.
Protests were also held in South Pacific island nations, which are among the most vulnerable to rising sea levels.
School children in the Solomon Islands, some of them carrying shields and flags and others in canoes, gathered near the sea to demand action to tackle the climate crisis.
Meanwhile, students in Kiribati, with their fists held high, chanted "we are not sinking, we are fighting", according to images posted on Twitter by environmental non-profit 350.
Swedish teen activist Greta Thunberg, who began the School Strike 4 Climate movement, sent messages from New York before the start of the demonstrations.
"It's early morning in the Pacific. Soon the sun will rise on September 20th 2019. Good luck Australia, The Philippines, Japan and all the Pacific islands. You go first! Now lead the way," she posted on Twitter.
"Incredible pictures as Australia's gathering for the #climatestrike This is the huge crowd building up in Sydney. Australia is setting the standard! Its bedtime in New York...so please share as many pictures as you can as the strikes move across Asia to Europe and Africa," she said in another tweet.
Demonstrations to push for climate change measures are scheduled to take place in hundreds of cities around the world on Friday.
The Youth Climate Summit will also be held on Saturday, ahead of the UN Climate Change Summit, where only countries that have taken concrete steps to improve the environment will take the floor.
( With inputs from IANS )