But the genial 22st (140kg) Samoan, one of the Premiership’s most improved forwards, has defended his switch, explaining how a significant proportion of his extra earnings will help maintain his family back home in New Zealand.
Johnston, 27, told The Rugby Paper: “It was definitely a hard decision because I owe everything to Harlequins. They’ve turned me into the player I am and I’ll always be grateful, but at the end of the day it was family stuff which made the decision.
“I’ve got a big family in New Zealand with two brothers and three sisters. But one of my sisters passed away and my dad has passed away as well, so my mum’s all alone now and it’s very important for me to send money back to keep her good.
“Mum’s on her own and my little sister has kids she’s trying to look after, so I try to help as much as I can. It’s very important to all of us Pacific Island guys playing overseas to send money back home and try to look after our families.
“My brother, Census, played for Saracens and says I’ll like it there. But at the end of the day it was a business decision and I want Quins fans to understand that.
“I’ve had a lot of stuff thrown at me on Twitter where people have literally said they hate me, called me ‘Judas’ and all that. People shout it at me as well, but I just hope they can understand and know that I’ll do as much as I can for the club before I leave.”
Johnston is clearly genuine when he says Quins have shaped his career and is lavish in his praise of how director of rugby, Conor O’Shea, and forwards coach, John Kingston, have turned him into a world-class performer.
After leaving the Ponsonby club in Auckland for National Two (South) outfit Richmond in early 2009, Johnston’s potential soon became apparent and he was snapped up by Quins, who went to work on the mental side of his game.
Ninety-nine games on, Johnston explained: “I came really raw to Quins but I’ve improved vastly as a player, especially with all the done with John Kingston.
“He’s changed my mentality towards scrummaging and made me believe I could do it at the highest level. I came into professional rugby not knowing what was going on but they’ve worked on the mental side, teaching me to concentrate from minute one to the 80th.
“In New Zealand, it was more about faster rugby – everyone wants to just run – whereas here there’s a lot more scrummaging and set-piece, which is what I now find really exciting.
“I’ve really enjoyed my time at Quins and the lads have been really good to me, so leaving is tough but it’s something I just have to do.”
Before he goes, though, there is unfinished Premiership business. Quins are lagging behind Saracens and Leicester and not assured of a play-off spot with two regular season league games left -although yesterday’s win over Bath puts them in the box seat.
And Johnston believes the letdown of last weekend’s Heineken Cup quarter-final defeat to Munster will act as additional motivation.
He added: “Last week was a huge disappointment but we are the champions and we’ve got to try and defend it. Winning the Premiership at Twickenham last year was one of the best days of my life and I’d love a repeat.”
That, Johnston believes, would be a fitting way to bow out.