It emerged that Jennings offered Espiner the 60 Minutes role over lunch at upmarket Wellington restaurant Pravda this month.
Espiner said he had mulled it over for 10 days, and decided the opportunity to work in a longer format current affairs show was great.
He said he was looking forward to finding stories about "real people" after more than a decade in the press gallery.
"Politicians are so guarded in their language and an answer is given for a vested reason and they have to toe the party line."
His good friend and TV3 political editor Duncan Garner had been a sounding board in the decision but "wasn't driving the boat".
His fiance Emma Wehipeihana has found a job at a recruitment company in Auckland.
McCallum won the best current affairs gong with reporter Mark Crysell at the Aotearoa Film and Television Awards recently for their story on the death of King's College student James Webster.
McCallum also had a second nomination for the same award.
During a long career at TVNZ, McCallum contracted the MRSA superbug while revealing poor resthome conditions in 2009.