THOUSANDS of robots in the ocean have hit a milestone with delivery of their one millionth piece of climate and ocean data.

The robots sit one kilometre beneath the surface of the ocean, where they are safe from ships and pirates and travel from two kilometres down to the surface gathering data.

When they reach the surface they send their data via satellite to those involved with the program.

Ten of the robots were first placed in the Indian Ocean in 1999 by Australia and now there are more than 3500 across the globe and known as the Argo project.

Argo co-Chair and CSIRO Wealth from Oceans Flagship scientist Dr Susan Wijffels said sea and climate observation networks were vital for gathering data for seasonal weather predictions.

"The 1982-83 El Nino event came out of nowhere and it surprised people becaue there was no observation system,'' Dr Wijffels said."

"Argo can help inform the climate modelling."

Dr Wijffels said Argo complemented climate data collected by the Tropical Moored Array in the Pacific.

She said the moored array could collect atmostpheric information including whether the weather was cloudy, but needed research vessels to service it.

Dr Wijffels said in the western part of the Indian Ocean the moored arrays could be vulnerable to pirates, because they floated on the water and were anchored to the sea floor.

She said the Argo robots were "about as tall as a person" and cylindrical and had a very low profile even when they reached the surface to send data to satellites.

Each one lasts four to nine years.

"They are largely made of metal and when they're no longer used they get crushed and fall to the sea floor,'' Dr Wijfells said.

She said there were more than 20 countries and dozens of agencies involved in the Argo program.

"They get different things out of it.  For example, India uses the information to predict moonsoons while others want to track the long term ocean temperature," Dr Wijfells said.