Terry A’Hearn slips quietly into the lush, cavernous lobby of a posh old Edinburgh hotel, joking about being allowed in.

The chief executive of the Scottish Environment Protection Agency, (Sepa), is casually dressed and beaming with boyish enthusiasm. He says that English is his second language: Australian is his first - after all he was born in Melbourne. As an environmental watchdog under fire, he is remarkably upbeat - almost as if he’s wagging his tail, but more on the canine metaphors later.

Sepa has come in for some serious stick recently over its relationship with big business, particularly the multinational fish farming industry. But A’Hearn is determined to demonstrate that he’s trying to save the world, one business at a time.

His big idea is “one planet prosperity”. He quotes research from the Global Footprint Network, an international think tank, suggesting that the world needs three planets to sustain its high levels of consumption and pollution.

“The most serious environmental problem is that our planet cannot support the economy and society that we’ve built,” he says. That means big changes have to happen, or humanity will pay the price.

“You can’t negotiate with the climate system. It will regulate us, whether we like it or not,” he points out. Overusing the planet’s finite resources “will seriously undermine our economic and social way of life in the medium to long term.”

So businesses have to transform the way they work to use fewer natural resources, cause less pollution and produce less waste. Since people don’t want to go back to living in caves, A’Hearn argues, “we have to get way, way smarter at using the planet.”

“I want the best environmental performers to be the most successful businesses,” he says. “Those businesses that use the environment less will make money. The ones that use it a lot won’t make any money.”

Those who ignore the reality of 'single planet prosperity' will be doomed, he warns. “It might be 20 years time, it might be 50 years time, but it will come. You won’t be carrying on if you use the environment to a huge degree because the environment won’t be able to support it. You won’t be able to sell anything.”

A’Hearn bridles at being called an environmental evangelist, but he accepts he’s on a mission, backed by science. His wake-up moment, he recalls, was when he was walking around the botanic gardens one evening in his home town Melbourne, Australia, in the late 1980s with headphones on.

He was listening to a talk that had been given by the renowned Canadian environmentalist, David Susuki. What shocked him was the realisation that the exponential growth in resource use simply couldn’t continue – and that the threats were real and imminent.

“This is unbelievably serious,” he says. “It changed my perspective on things. The things we talked about as being most important weren’t the most important.”

A’Hearn was born in 1963. He played cricket at school, became head prefect, and went on to study economics and accounting at Melbourne University.

After working for an accountancy firm, he joined the state government of Victoria, and then in 1993 its environmental protection agency. He left after 17 years to move to the UK, where he ended up as chief executive of the Northern Ireland Environment Agency.

In 2015 he moved to become head of Sepa, which he lauds as one of the world’s most respected environment agencies. “It’s not for Sepa to tell the people of Scotland what sort of economy or society they want,” he says.

“But it is for Sepa to say that the scientific evidence says the one you’ve got is taking three planets. There’s only one planet.”

A’Hearn’s strategy is to try and convince businesses to go “beyond compliance” by committing to cutting consumption and waste. In March he signed up the first company, Superglass Insulation in Stirling, to a “sustainable growth agreement”, and is aiming for another five this year.

His approach has caused critics to suspect him of being too business-friendly. “I’m friendly to good businesses,” he says, but stresses that he’s distinctly unfriendly to firms that break the rules.

“We will not tolerate people not complying,” he says. “If people screw the Scottish environment, we’ll take them to the cleaners.” His message for every businessperson he meets is the same: “If you screw up, we will beat you up.”

He has set Sepa the aim of making every business obey environmental rules. “I don’t know any other environment protection agency that says we will get 100 per cent compliance. That’s an incredibly difficult thing to do.”

Far from allowing standards to relax as part of 'one planet prosperity', there will be a “tightening and toughening” of Sepa’s role as regulator. “In five years time, compliance will be yesterday’s discussion,” he predicts.

A’Hearn defends his decision to drop plans to ban the fish farming industry from using a toxic pesticide after the industry complained. He says he didn’t take kindly to being put under industry pressure, and accepts that the internal emails published by the Sunday Herald “don’t look very clever”.

But he is confident that Sepa took the right decision: to reduce the pesticide’s use and wait on further scientific evidence before working out what to do next. Sepa should be receiving a new report within the next month or two, which would be peer-reviewed and published.

He brushes aside concerns about salmon farming companies buying him dinners, pointing out that he has also dined with environmentalists. “That’s a reasonable way of getting to know people and do business,” he says. “I’m a people-based person.”

A’Hearn was taken aback when Sepa was criticised for allowing the Scotch Whisky Association, which represents drinks companies, to help appoint senior staff. It’s arrogant to assume that the regulator always knows best, he contends, and it’s sensible to involve outside agencies.

Sepa’s job is to police industry at the same time as supporting it, he says. He compares the job of a regulator to the process of bringing up children: sometimes you need to punish and sometimes you need to cajole.

He also enjoys exploring suitable watchdog metaphors, describing himself as a “dog person”. Sepa is not a poodle, or a lapdog, or any kind of spaniel, he agrees. It’s not a golden retriever like the “surrogate child” he had to leave behind in Melbourne.

He eventually suggests a border collie as Sepa’s 'spirit animal'. “They sense if someone is friendly, decent and nice. They wag their tail. If someone’s threatening, they’ll bare their teeth. That’s what we’ll do.”

Outside, in the Edinburgh drizzle, A’Hearn poses for a photograph. He arranges his face into a toothy grin. Or it is a snarl? It’s hard to tell.