Lomu (40) died unexpectedly of cardiac arrest at his Auckland home yesterday morning after a long battle with kidney issues.
''He was a great man and a great rugby player,'' Mains said.
''He changed the way rugby could be played. It is a huge loss.''
Lomu had been in the United Kingdom as a spokesman during this year's Rugby World Cup.
He and his family had arrived back on Tuesday night.
Long-time former All Blacks doctor John ''Doc'' Mayhew said Lomu's kidney issues would inevitably have been related to his heart stopping.
''The final mechanism was something caused the heart to go into cardiac arrest, most probably a cardiac or pulmonary event,'' Mayhew said.
Lomu had battled a kidney disorder since 1995 and had a transplant in 2004.
In 2011, he had a health setback when his body rejected the replacement organ.
Lomu scored 37 tries in 63 tests for his country but those raw statistics do not do justice to the impact he had on world rugby.
Otago rugby legend Mains was instrumental in shaping the young loose forward into the most potent winger in the game and told the Otago Daily Times he was ''devastated''.
Mains first became aware of Lomu's prodigious skills in 1993 through Otago rugby stalwart Des Smith.
''Des Smith, who had been manager of the New Zealand secondary schools team, had said to me keep an eye out for this kid.
''We saw him playing at the nationals sevens tournament. I was with [selector] Earle Kirton and I believe we looked at each other and said what are we going to do with this kid, because he was actually a loose forward at that stage.
''We looked at him and said, as a loose forward he is not going to be much bigger than the guys he is playing against. But if we turn this kid into a wing, he could turn out to be dynamic. History speaks for itself.''
Lomu made his debut for New Zealand against France in Christchurch in 1994 but it was at the 1995 Rugby World Cup in South Africa where he became a global sporting star.
He scored the sorts of tries previously only imagined by children and even caused experienced commentator Keith Quinn to botch his lines in the most memorable way: ''Lomu, Oh, Oh,'' summed it up, really. He took our breath away.
''A large part of [our game plan for the 1995 World Cup] was centred around getting the ball to our wing three-quarters - we had two brilliant ones, of course,'' Mains said. [The other was Jeff Wilson].
''But Jonah was the main focus - and getting the ball to Jonah with space to move in. The faster we could recycle ball and get it to Jonah, the greater the opportunities we had.
''One of the great strengths of Jonah's play was, if you put him in space, he would score a try or take two or three of the opposition defenders out and then offload the ball.''
Lomu could beat players on the outside, inside or just run over the top of them. But shortly after the tournament he was diagnosed with nephrotic syndrome - a serious kidney disorder.
Mains found out just before the All Blacks left for their end-of-year tour.
''It was a matter of having one or two of his mates around the team work with him in controlling his food and getting some extra work done because he turned up for that tour not very fit, and we knew the reason why.
''It was just amazing that he could achieve what he achieved on basically a six-cylinder engine running on four cylinders. It was incredible and just how good would he have been had he not had this illness?''
Mains said Lomu remained humble ''despite all those great things he did at the World Cup and that was a measure of the humility of the man''.
''Off the field he was always a very placid, pleasant individual to have around and willing to speak and communicate and mix with anybody.''
Lomu is survived by his wife, Nadene, and sons Brayley (6) and Dhyreille (5).
Nadene issued a statement yesterday afternoon.
''It is with great sadness that I must announce my dear husband, Jonah Lomu, died last night. As you can imagine this is a devastating loss for our family.''
She asked for privacy for her family, especially her children, during the ''traumatic time''.
Members of Lomu's extended family gathered at home in Mangere to support each other following the news.
Prime Minister John Key said ''the nation will be devastated with the news that we've lost Jonah Lomu''.
''He will be remembered not only for being a truly amazing global sportsman and for his time on the field [but] for his sportsmanship.''
- Additional reporting The New Zealand Herald, NZME.
