He wrote it less than a year after conservative UK Prime Minister Margaret Thatcher became the first world leader to speak strongly about the dangers of global warming, in a speech warning that "we have unwittingly begun a massive experiment with the system of this planet itself".

Mr Miller found the Warners Bay High School newsletter in his garage last week after Australians died in catastrophic bushfires that destroyed thousands of properties, millions of hectares of bush, more than one billion animals, and left images that horrified the world.

He went looking after Swedish teenager Greta Thunberg shared a photo on Facebook of a kangaroo silhouetted against a wall of flames, and slammed Australian politicians for climate change inaction.

"There were a lot of people attacking Greta Thunberg," he said.

Mr Miller, who was in the first intake of environmental management students at the University of Newcastle in 1990, said reading his poem more than three decades later was confronting because "morally I've failed".

"I thought I was going to be the first of these environmental scientists to come through uni and I was going to go out and save the world. Instead you get distracted on your own things," Mr Miller said.

"I was involved in environmental work but I got caught up in more localised, Australian-based issues, and the global issues weren't really in the front of my mind."

His personal experience reflected Australia's experience of climate change warnings, he said.

"Like most people I just turned a blind eye to climate change. I just thought it would sort itself out, that we'd develop some sort of technology to deal with it.

"When we had recessions the environment was not a priority. When the global financial crisis happened, the environment was not a priority. We've totally dropped the ball on this."

Mr Miller said critics who might try to dismiss him as anti-coal will have problems. He was employed until 2018 with a national company working directly in coal mining. Images of him still appear on safety posters and in films across his former employer's network. He has many friends who continue to work in the coal industry across the Hunter.

But he supports Hunter students striking for climate change action, speaking out against planned Hunter mine expansions, and pushing for state and federal governments to lead on a transition plan for the region, that acknowledges the risks climate change poses for the Hunter economy.

"A lot of people have been attacking Greta Thunberg. My view was that she sounded like a bit of a nutcase but then I looked at what she was actually saying. She's pretty much just focused on climate emissions, global warming and the need for urgent action now," Mr Miller said.

"I feel a lot of respect for what she's doing because she's right. People have attacked the student strikers but if noone says anything, nothing changes.

"It's a powerful message if kids are out there saying it's their future we're playing with. They'd like to live a better future than fighting fires and picking up broken houses after cyclones. People keep saying we've had bushfires and cyclones before but what they're missing is that because of global warming, they're occurring more often and at much greater intensity, as we've seen with these bushfires, and that's only going to worsen if we don't act now."

Greta Thunberg is "just as passionate as I was back when I wrote the poem", Mr Miller said, but three decades later the future he warned about is becoming her reality.

As a year 12 student in 1989 Mr Miller wrote that "We've recklessly broken all the rules/ With the continued burning of fossil fuels/ The superpowers need not take sides/ Everybody will drown from the rising tides."

He predicted "Farmers' land... turning into a dustbowl"; water "no longer fit to drink"; toxic waste and "total ignorance" leading to "floating fish". He despaired at a "political bungle" in South America leading to the "selling-off of the Amazon jungle" where "they're slashing and burning nearly everything".

In the final stanza Mr Miller called for action with: "Scientists say we should still have hope/ But that's as hard to grasp as a wet piece of soap/ We still have to fight hard for our survival/ This will require a "Greenie" revival/ Help me live to 93/ Get out there, get chained to a tree."

In the 30 years since Mr Miller's poem the price of coal has peaked and troughed from $US37 a tonne in 1989, $US21 by 2000, up to $US60 by mid 2004 at the start of the mining boom and peaking with an all-time high of $US180 a tonne by late 2008. It has peaked and troughed since then to sit in the mid $US60 a tonne for the second half of 2019.

Mr Miller acknowledges the Hunter economy, and the fortunes of tens of thousands of people in the region, have turned on those peaks and troughs, which is why he backs calls from within the Hunter for governments to act now on a transition plan away from coal.

The lack of a transition plan leaves the Hunter at risk, said Institute for Energy Economics and Financial Analysis director Tim Buckley, a former leading analyst with some of the world's biggest financial institutions.

"The very fact the NSW Government has no transition plan means we're leaving the future of the Hunter Valley up to decisions of individual foreign companies to manage that transition," Mr Buckley said.

Mr Miller supports Hunter students who have turned out in their thousands wanting Australia to reduce emissions in line with the Paris Agreement goal of restricting future warming to 1.5-2 degrees above 2005 levels.

He supports students who recently called for Australians to make submissions opposing Glencore's plan for a major expansion of the Glendell mine at Singleton until 2044, and more than doubling annual production.

Mr Miller acknowledged the thousands of people directly employed in the mines, the many thousands of businesses across the region that rely on the mining industry, and more than two centuries of coal as a foundation for the Hunter's economy. But future mine expansion is a moral issue, he said.

Climate science requires global carbon emissions to be reduced to zero over the next three decades, and expanding Hunter mines rather than closing them when approvals expire will prevent that, he said.

"The size of the expansions shows the companies are trying to get as much coal as they can, as quickly as they can, to make as much money as they can, but given what we know about the consequences of that coal being burnt, morally that's not good," he said.

On the weekend Prime Minister Scott Morrison said Australia was going to "evolve our policy" and "reduce emissions even further" than the 26 per cent higher than 2005 levels by 2030, which is said to be one-third of what's needed to keep global warming to 1.5 degrees.

Mr Morrison said any changes would be made "without shutting down traditional industries upon which regional Australians depend for their very livelihood".

Mr Miller said he expected to be criticised for speaking up, but a majority of Australians understood that today's needs are not the only consideration.

"We could have done something to prevent this and didn't. It's time we did now," he said.