But by the time they realised how it badly it was affecting them, it was too late.
The pair could ''easily'' smoke four or five bags a day - each.
At $25 a packet, they estimate they spent $7000 on the products just this year.
Their lives followed a pattern of eating, using legal highs and sleeping.
At their lowest, they were collecting bits of rusty metal from the foreshore, taking them to scrap yards for money - they would steal anything to fund the habit.
After a two-and-a-half-year addiction, Nathan's health has seriously deteriorated and about three weeks ago, he vowed to ditch the habit.
Suffering kidney problems after four years of using legal highs, his friend Aaron decided to quit with him.
The cost of their addiction was highlighted by Nathan's father, Neville Bryant, who, in front of them, counted out $5882 worth of legal high packets he had found in the house.
''I didn't believe Dad when he told me,'' Nathan said.
''I could've bought a car for that.''
SGT-24, Voodoo, Karma, Anarchy, Kronic - they have tried almost everything that has been available in Oamaru.
Under the influence, Nathan was verbally abusive to his parents, demanding money to buy more.
He had been particularly intimidating towards his mother, Helen Belcher, who had to try to hide from her son at times.
''I blame myself for some of it. He used to get quite abusive, wanting money, and I was scared and I'd give him money,'' Mrs Belcher said.
''Afterwards I'd feel like kicking myself because I'd given him money when I shouldn't have, but you were just so scared in the moment.''
Several times she thought about contacting Linda Forbes, who organised several protests against the sale of legal highs in Oamaru, but was scared of the fallout.
His parents say they tried to get their son to stop using legal highs.
''We tried bribery, we tried everything - it didn't make a difference,'' Mr Bryant said.
The drugs had made Nathan very ''confused'', and his parents worried the legal highs would have long-term effects on the 29-year-old.
Despite the withdrawals, they have already noticed a difference in their son.
''It was like there was a different person in the house - there wasn't the firing up at us, wanting money to get his daily fix. The pressure was gone,'' Mr Bryant said.
''He even seems happier, even though he's going through these massive withdrawals.''
Aaron (19), who lives with Nathan's family, is not having the same withdrawals.
He still has problems with his kidneys, but on the whole he is feeling healthy, has a new job and is determined never to touch the products again.
The pair say they have lost many of their friends since ditching the habit, but they are placing priority on their health and their future.
Mr Bryant and Mrs Belcher could not be prouder.
- Rebecca Ryan
