"I take the view that this is a medical condition and I should just go ahead and treat the person and their condition with the same concepts and therapeutic alliance that I would with someone with anything else."
Farrell, who co-owns Mount Medical Centre, has been a GP for 25 years and has specialised in addiction since 2013 having treated "hundreds" of addicts.
He says that the number one culprit causing drug harm in New Zealand is alcohol, largely due to advertising, price and availability.
He fears that sophisticated marketing could also happen to cannabis if laws are liberalised and, combined, alcohol and cannabis increase each other's effects.
"We know that there's at least 100,000 alcohol dependent patients in the country and one in 10 people who use alcohol are going to have some degree of problem," he says.
In 20 per cent of GP consultations, drinking is a factor in why a patient presents - be it stomach or stress issues.
Farrell treats mostly men – with an average age of 27 - but he sees female heavy drinkers too.
"Some are functional alcoholics, just quietly wit-scared that they can't stop."
If someone has a "severe" addiction in their mid-20s and they don't stop, his observation is that by the time they're in their mid-50s, they're suffering serious health issues or they're dead.
"If you've got addiction, you might have a fight with your [partner] but: 'They'll get over it', and then you might have a near-miss car accident: 'Oh, that was lucky'.
"You're not quite suffering enough consequences to make you go: 'It's my drinking'.
"Functioning alcoholics often lead orderly but kind of robotic lives, compared to the chaos of a severely dependent drinker.
"But what they're quietly doing is damaging their brain, impairing their sleep, increasing their risk of suicide, depression, heart disease, arthritis and all sorts of other things and they're just doing it slowly."
What people need to choose, instead of drugs, is connection and mindfulness, says Farrell.
"Being engaged in their life with people around them."
He feels disappointed that the Mental Health and Addiction Inquiry overwhelmingly focuses on alcohol but the Government is yet to do anything about it.
This is also despite $7.8 billion in alcohol-related damage, reported by Berl principal economist Ganesh Nana last year.
"That's 43 per cent of our health budget per annum," Farrell laments.
"I really believe we should have an institution like the Reserve Bank for drug policy so politicians couldn't come in and say: 'Let's be tough on drug crime and then do some policy that's going to create a lot of damage'.
"That's the moral injury.
"It's not hours per week that gets you stressed, it's what could be done."
The making of a doctor Farrell grew up on a sheep farm in Hinds, 15km south of Ashburton.
He got top marks at the Catholic boarding school he attended and at 17, was getting a lot of pressure to become a priest - which he declined. He adds that becoming a doctor wasn't an "innate passion" either.
After completing his fifth year of medical studies at Otago University, he took a year off and travelled overseas.
"I came back and did my house surgeon years and my lovely wife [a psychologist] got a job at Tauranga Hospital."
That was in 1991 and they couldn't afford to buy a house in Tauranga so they bought in the now-pricey Mount Maunganui, which he calls "dumb luck."
To combat stress, the father of three is learning to play golf and enjoys woodworking. It's a skill that's methodical but he wouldn't describe himself as such.
"I'm quite impulsive. I guess I think laterally a lot and do a lot of problem solving but that's one of my weaknesses - I charge in too fast."
His passion for helping addicts stems from his university days, when he saw violent assaults fueled by booze.
"Later, as a GP, I was very mindful of the impact of lifestyle on disease.
"We were trained in prevention so I would ask people about smoking and drug use and that included all drugs. I learned to be non-judgmental."
Close friend Dave Halligan says the likeable Farrell goes over and above.
"Tony knows that to establish a relationship, he has to spend some time with people and he's an empath.
"He comes across as someone who is on the same level as you, talking in your language.
"He's got that humanistic way of doing things and maintains a sense of humour but at the same time, he's very technically-minded."
Farrell has remained committed to the challenges of helping addicts and mental health patients, despite the fact he's fielded a punch to the face, had death threats, and talked patients into surrendering knives.
His job isn't for the fainthearted and it isn't well resourced, unlike other GP specialities like skin surgery.
"My special interest doesn't really attract the same level of regard," he says.
"I think it's been said that psychiatry is the Cinderella of medicine."
He became authorised to write methadone scripts in 1997 for opiate dependent patients and then did a postgraduate certificate in addiction medicine, before getting a fellowship 2003.
All GPs with a special interest must get Medical Council of New Zealand vocational approval and specialists are low in numbers due to GPs being already overloaded with work.
There are five GPs in the country with a fellowship on addiction medicine and one in training.
It's a low number, given the rate in which Kiwis are poisoning themselves with drugs.
It's been reported that there are over 700,000 people in New Zealand with high-risk drinking alone.
Farrell operates a shared-care service for methadone patients from his clinic, works on alcohol assessments for Civil Aviation and driving assessments for those with recurrent drink-driving offences.
He is a medical spokesperson for Alcohol Action NZ; chairman of men's family violence group Rongoa Whanau, board member at mental health and addiction centre Hanmer Clinic, helps train young doctors, and is on the national implementation committee for Hepatitis C, which is charged with finding and treating New Zealanders who have the chronic disease, which can take 10 years off your life.
Fifty-percent of sufferers (24,000) are still unaccounted for and treatment which has proved curable is fully funded.
"Infection relates to being exposed to blood and generally by way of injecting drug use.
"Often people have only had one episode of that," he says, giving the example of exploration at university.
"There's a lot of professional people amongst others who may have it and don't know they've got it, and so we want to encourage people to come forward if they have risk factors."
Farrell says there needs to be much better education around drugs.
The root causes of addiction are often associated with other mental illnesses and childhood trauma.
Furthermore, when it comes to the rise of methamphetamine use, he has thoughts on the judgment released by the Court of Appeal in October that states meth dealers who can prove their own addiction caused their drug offending could have their sentences cut by 30 per cent.
He says imprisoning addicts will likely make them worse, because of their mental health and isolation from their supports.
"However, we've got to be really careful in our field not to medicalise bad behaviour.
"I just wonder how they're going to determine was it addiction that led to the [crime], or did getting involved lead to sales and then addiction? How are they going to know?
"If 100 people take methamphetamine, probably a good 70 per cent can contain that and 30 per cent just can't."
I note that 70 per cent containment is quite surprising.
"Well, that's kind of the media portrayal … 'One shot and you're gone'," he says.
"You need six months of the symptoms of addiction to meet criteria for diagnosis.
"There's a lot of dissonance and incongruity in the way drugs are looked at and there's this kind of dichotomy that if it's illegal it must be bad.
"It's all about dose and frequency… Many people haven't looked at the history or the facts about each drug."
He gives the example that LSD and psilocybin magic mushrooms are being used in micro-dosing trials to improve human psychology; and in the 1970s, methamphetamine was prescribed in New Zealand as a weight loss product.
Cannabis was sold as tinctures in pharmacies in the early 20th Century.
He supports Labour's drug testing bill which was blocked by New Zealand First. Their youth group asked them to rethink that proposal to drug test at festivals because it's a good chance to provide drug education and save lives.
"Drug education often promotes: 'Don't use drugs', which is hypocritical, because everyone uses alcohol, well 85 per cent, so it's immediately stupid.
"I've seen this picture of a couple of fried eggs and it says: 'This is your brain on drug education'. So drug education needs to be factual.
"The best way to reduce harm is to not take drugs but if you do decide [to], this is the info you need.
"The alcohol companies don't give us the info we need to make decisions about alcohol," he says.
There's no cancer warning or: 'Hey, you might get violent and kill someone on this'.
"It gets special treatment when in fact it's a dangerous drug, and should have a more careful treatment."
Most of Farrell's patients who battle addiction are doing great and some fall off the wagon every now and again but he never gives up.
"I get back in behind them and say: 'Hey, I know you're feeling really bad about yourself but look, if you had cancer and it came back, we'd treat it wouldn't we?'
"I've been really, really glad to be able to help a lot of people with addiction because they're really grateful."