He also appears to have lived a life with filled more with humbug that hallelujah, and is described by someone who knew him well as a "classic Scrooge" who reused teabags to save money even when it was now apparent his bank balance was into nine figures.
The friend noted that even after quietly becoming one of New Zealand's richest men - he would have ranked around 130th on the NBR Rich List, but escaped their notice - Plumley still insisted on buying second-hand cars.
"He still had the first dollar he ever earned, but didn't frame it because he wouldn't have wanted to pay for one," the friend said.
While raised Catholic, and having attended Sacred Heart College when it was located in Grey Lynn, Plumley was said to be only a sporadic church-goer who for most of his life preferred to spend his Sunday mornings on the greens of the Auckland Golf Club.
His funeral in August 2016 was notably billed as a "celebration of life", instead of a formal Catholic requiem mass, and was held not at a Catholic church but at the Auckland-Council-run non-denominational Manukau Memorial Gardens Chapel.
His memorial notice quoted a line from the poem Elegy Written in a Country Churchyard - a 1751 classic written by Thomas Grey where the churchyard in question is the Anglican St Giles parish church in Buckinghamshire.
Despite this, those who knew him say he was open in his latter years about his intention to donate his fortune to the Catholic Church, and his will was clear.
His motivation was apparently less a drive to do good works, and more about cutting his step-children - Marshall's children from an earlier relationship - out of any inheritance, a friend said.
"I'm not entirely sure why, but he hated her children with a passion, and set the whole thing up so that she would be looked after while she was alive and that they didn't get any money," the friend said.
"I would say the main reason he gave it to the Church because he would have hated to think anyone would have had fun spending it."
Weekend Herald attempts to reach Plumley's step-children, who are understood to have eventually received some inheritance after their mother Marshall disputed the will but died aged 75 shortly after settling with the diocese, were unsuccessful.
Lyndsay Freer, spokeswoman for the diocese, said of the dispute: "The parties involved came to an amicable and satisfactory arrangement, and they signed an agreement that the matter would remain confidential to them."
The size of the donation to the diocese - to date $116m in cash and $4.5m in farmland has been paid over, with another $2m expected in the imminent wash up from Plumley's estate - is unprecedented and it is believed to be the largest charitable donation in New Zealand history by some margin.
Late last year the University of Auckland - the largest registered charity in New Zealand - trumpeted $16.5m given by the Hugh Green Foundation as the largest donation it had ever received.
Michelle Berryman, the executive director of the Fundraising Institute of New Zealand, asked if - to her knowledge - Plumley's bequest was the largest donation the country had seen, replied: "Oh god yes."
This staggering bequest only came to light after a Weekend Herald investigation into the finances of the religious sector picked up an enormous - nine-figure - surge in bequest payments to the Catholic Diocese of Auckland in 2018.
The investigations revealed that even before the Plumley donation the Catholic Diocese of Auckland was already the richest religious organisation in New Zealand, and the bequest pushed its net assets - largely comprised of central city real estate - to $1.07b.
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The Plumley donation has to date largely been invested, with diocese general manager James van Schie saying it was easily the largest bequest the Church in New Zealand had received and effort was focused on creating a Harold Plumley Endowment to ensure it became an ongoing source of funding for the Church.
"We recognise this is an extraordinary moment, and we don't want the intent of the donor lost in the sands of time," he said.
When his donor was compared to Scrooge was raised, van Schie said "I'm not a biographer of his life," but noted Plumley's late mother Mary had been a devout Catholic and he appeared to have become a more regular mass-goer later in life.
The bequest did not come with confidentiality conditions, but hadn't been publicised - outside of being recorded as part of a consolidated revenue line in annual accounts - largely in keeping with the character of the man who gave it, van Schie said.
"There is no secrecy about it, but he wasn't seeking accolades in life."