"He should've come to TVNZ 15 years ago when I offered him a job, but never mind."
Ralston said the job was for a 7pm current affairs show.
Asked what Campbell said to him at the time, Ralston laughed, saying: "He dragged it out for a couple of weeks and I think he got a very big pay increase off [then news chief] Mark Jennings at TV3 and then sent me a dozen wines saying sorry.''
History aside, Ralston believed TVNZ would benefit from Campbell's versatility.
"I think it just adds to the overall power of their news and current affairs product.
"John does excellent documentary-style stuff, investigative stuff, he's a hard-line interviewer and he's a good all-round player. They're looking pretty good with him on board. That's really impressive."
Campbell's move to the state broadcaster would also attract more audience.
"What they've done is really extend their reach,'' Ralston said.
"They haven't had roving correspondents, who can work across a number of programmes, for the last 15 years.
"Now they've got someone who can do stuff for Sunday, do it for the 7 o'clock programme, go on the Q+A programme.
"They can move him around and he can do specialist stories that, frankly, TV3 won't have."
Ralston said it was a "remarkable achievement'' that the appointment was kept under wraps until the announcement was made this morning.
"They played this one pretty close to their chest, considering it went through TVNZ and RNZ."
TVNZ confirms roving reporter/presenter role
Campbell will leave Radio NZ to join the state broadcaster in September and will file for news and longer pieces for the Sunday programme.
The appointment means that TVNZ now boasts two of rival network TV3's former biggest stars.
Hilary Barry is a co-host of Seven Sharp - and it appears the duo will again appear together on screen, with Campbell likely to fill-in on the 7pm show.
Campbell is also likely to step in to present Tonight, Breakfast, Seven Sharp, Sunday, Q+A and 1 News, the state broadcaster said.
TVNZ's head of news and current affairs John Gillespie said Campbell would be "a high impact player for 1 News."
"John's a broadcaster known for his love of superlatives so let me return the favour.
"The calibre of his work is fantastic. He tells stories that matter, he delivers robust journalism with a lot of heart and he breaks stories that draw audiences in. We'll be amplifying that."
Campbell said he was looking forward to his new role, which would see him split his time in and out of the studio.
"After 20 years of sitting in a studio five nights a week, I'm really excited by the prospect of spending more time in the field," he said.
"I love getting out, meeting people and sharing their stories. I can't wait to get to work."
Campbell, the former host of Three's Campbell Live, has hosted the Radio New Zealand Checkpoint show since 2015.
Last year he co-presented What Next? for TVNZ and another series is planned.
RNZ told staff Campbell's departure was a "big loss".
"John has done terrific, pioneering work since joining RNZ in 2015. He will leave a legacy of high-quality, innovative journalism and improved audience engagement," chief executive Paul Thompson said.
Campbell would continue to 'bust a gut on Checkpoint" till he departed the company.
Thompson said Checkpoint would continue in its current format.
"Checkpoint will continue as an Auckland-based multimedia news and current affairs programme under a new presenter," he said.
"Checkpoint has been a flagship show for RNZ for more than 50 years. Long may that continue."
Campbell's close friend and former co-host Carol Hirschfeld left RNZ under a cloud earlier this year after repeatedly misleading bosses about a meeting with Broadcasting Minister Clare Curran.
