But what followed was an "unmitigated trail of deceitful lies and manipulation" which "destroyed a massive amount of lives".
He described a web of deceit involving fake contracts and email trails which appeared to be legitimate.
At first everything seemed to be going as planned. The victims put in money and received payments back after jobs were completed. But towards the end of 2017, the payments were getting late, and the excuses started coming in.
The victim discovered Johnston was copying fake email accounts into emails between investors.
Once he realised what was happening, he reported the crime to police.
It has since been revealed Johnston received at least $3.2m.
The victim lost $200,000, though he was owed nearly $500,000 in payment. Between himself, his brother, and his friend, the trio lost almost $2m.
"The other friend, he lost his complete retirement savings," the man said.
"It's done major damage to personal relationships and family relationships."
He said there was an "element of guilt" on his end because he had been the first person to invest in Johnston's business and others had followed suit.
"He's just a deceitful low-life liar that will manipulate anybody in any situation for self gain."
Johnston had "many opportunities to come clean" but instead "just carried on blindly".
Johnston had emailed the victim many times since police began investigating him, but the victim refused to respond, and declined a restorative justice meeting.
"He's trying to be sympathetic, feels sorry for me, 'I've got a problem, I'm sick, I've got a gambling problem', that sort of thing."
The victim's wife said her husband's health had suffered since the offending was revealed, among other things.
"It's been a very stressful 18 months . . . [he's] certainly not the same person he used to be. It's caused lots of stress on, I suppose, our marriage, stress on his relationship not only with myself but with his brother and with his best friend of 20-odd years."
Financial Markets Authority spokesman Andrew Park said the "key thing" for people to remember when someone was asking them to invest money into their business was "know what you're investing in".
"You've got to take extra caution if someone is asking you to invest in a private company," he said.
"If people are thinking of investing, look at the list of products that are regulated by the FMA.
"Press pause when someone's asking you to give them your money, and check that it's all legitimate or licenced by the FMA."