Mr O'Sullivan worked in sales for the legal-high manufacturer and distributor from 2009 until he resigned in October 2013.
There was dispute about whether he was an employee for all that time or whether he acted at times as an independent contractor, but the authority ruled he was an employee of the company.
Mr O'Sullivan told the authority problems with tax payments and the non-payment of a $500,000 loan contributed to his leaving the company.
Ms Campbell said the authority had no jurisdiction over the loan but ruled the company breached good faith obligations over the unpaid tax.
The tax problems arose because Mr O'Sullivan was in April 2013 owed commission payments by Stargate of $798,0000, of which $512,000 would need to be paid to the Inland Revenue Department.
It was agreed Stargate would give that amount to Inland Revenue, which it had not done when the authority hearing was held in May.
By October 2013, Mr O'Sullivan resigned from the company, citing the appointment of a new general manager.
"So I think it is best if I hand in my notice now so you both can run the company without any interference and I can move on," he wrote to his managers.
"This isn't me throwing my toys out of the cot as I have had time to think it over... I would like my tax sorted out before I leave. [Stargate] has had over 500K of my tax money for over a year now and done nothing with it... which has put my family at a lot of risk over the last year."
However, he subsequently claimed he was unjustifiably dismissed. The authority didn't agree, nor did it uphold counter-claims against Mr O'Sullivan, with no-body from the company turning up to the hearing.