At the time of the walkout, ministers had been discussing in “Meeting Room 3” in the COP venue to thrash out details of a deal on more cash, even as the talks ran more than 18 hours over time.
Mr Robertson said the walk-out was “spontaneous”, not planned. “It was so quick and instinctive,” he said.
“Evans Njewa [the chair of the Least Developed Group] raised his country flag. He took the floor and explained how frustrated he was,” Mr Robertson said.
“‘You’re not hearing us’, he said. Then he stood up. Everyone stood up with him and then we just left,” he added.
Journalists on the outside of the meeting room saw the door open, and the group of negotiators walked out.
Mr Robertson said the two groups went next door into “Meeting Room 11”.
“As soon as we did that everyone was calling us – the COP presidency, developed counties, the chair of G77 [another powerful bloc of developing countries],” he said.
“It’s really sad that it took us walking out to get that,” he said.
He said that AOSIS were unhappy that their calls for more ring-fenced money was not listened to.
“The size of the damage to our economies [from climate change] outweighs any sort of per capita calculation of how much money we’re getting,” he said.
He referred to the impacts of hurricane Maria in 2017 on the Caribbean island Dominica.
“It wiped out over 200% of its GDP. You can’t say that for many other developing countries,” he added.