Swedish teen activist Greta Thunberg will set sail across the Atlantic tomorrow to a major UN climate meeting in Spain thanks to two Australians.

On Wednesday, Ms Thunberg said on social media a group, including Australians Riley Whitelum and Elayna Carausu, offered to help her make the voyage.

"So happy to say that I'll hopefully make it to COP25 in Madrid," she said.

"I've been offered a ride from Virginia, USA, on the french 48ft sailing catamaran La Vagabonde. The two Australians Riley Whitelum, Elayna Carausu and Nikki Henderson from England will take me across the Atlantic Ocean.

"We sail for Europe tomorrow morning!"

READ MORE Hillary Clinton says male leaders are 'scared' of Greta Thunberg

The 16-year-old had been seeking an environmentally-friendly way to travel from the US back to Europe.

The COP25 climate meeting was due to be held in Chile but was moved to Spain at the last-minute after the South American nation was hit by protests.

According to a New York Times profile, Mr Whitelum and Ms Carausu have spent several years sailing the world, with a newborn son being a new addition.

"More than a million people subscribe to their channel, Sailing La Vagabonde [the name of their boat], which has chronicled their life aboard in endearing, instructive and sometimes terrifying videos," the profile says.

"[There have been] two Atlantic and one Pacific crossings; maggoty trash; broken equipment; storms and becalmings; scaldings and other injuries; the boredom of weeks offshore when you’ve read all your books; would-be pirates; and this year, a stowaway, their 10-month-old son, Lenny."

READ MORE Greta Thunberg, Leonardo DiCaprio unite in climate crusade

Ms Thunberg had earlier sent out a plea on social media.

"As #COP25 has officially been moved from Santiago to Madrid I'll need some help," Ms Thunberg tweeted.

"It turns out I've traveled half around the world, the wrong way:)"

"Now I need to find a way to cross the Atlantic in November... If anyone could help me find transport I would be so grateful," said the teen, who refuses to fly because of the carbon emissions involved."

Ms Thunberg's highly publicised journey has so far involved crossing on a zero-emission sailboat from the coast of England to New York, traveling overland through North America by train and in an electric car borrowed from Arnold Schwarzenegger.

The UN announced Friday that COP 25 will take place in Madrid, on the original scheduled dates of 2-13 December.

Ms Thunberg rose to prominence last year after she started spending her Fridays outside Sweden's parliament, holding a sign reading "School strike for climate".

Students across the world began emulating her campaign, leading to organised school walkouts and the rise of the "Fridays for Future" movement which targets government action on climate change.

Additional reporting: AFP