Three of the charges relate to further fraud Johnston committed while on bail for the earlier offending.
Defence lawyer Mike Antunovic said Johnston accepted the total amount he had received through his fraudulent activities was close to $3.5 million, and the resulting loss to his victims was more than $2m.
The 35-year-old, who has no previous convictions, was sorry for the "significant harm" he'd caused to the victims - one of whom lost $1.4m in retirement savings.
Antunovic said it would be pointless to make an order for reparation, as Johnston had no money.
According to a report by forensic psychiatrist Dr Justin Barry-Walsh, Johnston's offending was "disordered" and "difficult to control", Antunovic said.
"His life has been dominated by the need to gamble almost on a daily, hourly basis for a number of years," he said.
"It has consumed most of his time. What's he's done hasn't made him happy, and hasn't made him wealthy, and now he's in jail and he has nothing."
Antunovic said Johnston acknowledged the "tragic and severe consequences" of the offending not only on the victims, but on himself and his family.
Judge Jan Kelly said Johnston "in essence operated a Ponzi scheme" to deceive his victims.
His family, who were in court to support him, provided a letter to the court, which revealed he had had an "excellent education" and was involved in sports and sports clubs, and had completed a bachelor of accounting and economics at Waikato University.
The total amount of money he gained through the offending was $3,416,965.
"It was not a rash or impulsive decision to offend, it was repetitive," Judge Kelly said.
She had "reservations" about the remorse Johnston had expressed.
"I have concerns of the minimisation of the effect of your offending on what you perceive to be wealthy victims. Also, it seems that your remorse is more directed to the situation you find yourself in and your desire to avoid prison."
The victim who lost his retirement savings earlier told the Herald that losing that amount of money was "horrific".
"Your life changes in an instant. It's devastating, you can't even explain it. All that money gone. You can't imagine mentally how you'll feel after that.
"Sixty-five years old and I've lost $1.4 million," he said.
"I don't know if I'll get the money back. I will never see $1.4 million.
"You don't recover from that unless you win the lottery."
He said he'd suffered severe health issues since the offending, including getting ulcers and having to go on anti-depressants.
Another victim said the offending was an "unmitigated trail of deceitful lies and manipulation" which "destroyed a massive amount of lives".
"He's just a deceitful low-life liar that will manipulate anybody in any situation for self gain," he said.
All eight victims have permanent name suppression.
Judge Kelly sentenced Johnston to five years in prison. She did not order reparation as Johnston did not have the means to pay it.
