With 60 per cent of his customers based internationally, and worldwide recognition as a respected stamp dealer, Mr Mowbray is responsible for world record stamp auction sales.
In addition to his usual trading, he is busy preparing the 29th annual Major International Stamp Auction, taking place in Wellington next month and set to comprise $1.6m in estimates.
His role, he said, is a mix of hobby and business and quickly became his "dream job".
"I started collecting stamps as a child, when my father gave me a box of stamps to shut me up one day. I began swapping them at school and liked the trading more than the collecting. I eventually became a dealer and now I'm not a collector as such."
Among his most significant sales was a sheet of reconstructed 1840 Penny Blacks stamps during an auction in Wellington, which sold to an international collector for $88,000.
Issued in 1840, the Penny Black was not only Britain's first postage stamp, but the world's first.
Another notable success was the sale of a 1947 Threepenny Vanguard stamp in March this year, an unissued stamp from the 1947 royal visit, which is now cancelled.
With only seven of the stamps believed to be left in the world, it sold for $67,850 to an overseas collector and outstripped the previous world record of $31,050, set by Mr Mowbray in 2009.
As well as having founded the New Zealand Stamp Dealers Association (NZSDA), Mr Mowbray was elected president of the International Federation of Stamp Dealers Associations, from 1990 to 1992.
Prior to that, in 1978, he became the first member of the NZSDA to hold an auctioneer's license and, in attending the 1980 London World Stamp Exhibition, his company became the first New Zealand stamp dealer to have a stand at an international exhibition.
A year later, he won the Air New Zealand Enterprise Award for the Wellington region.
Stamp collecting, he said, suits people wanting a relaxing, slower paced hobby that they can do privately, as they like and without any rules.
"I've done this for so long that people from all over the world know and trust me, and ship stuff to me.
"They know of me as a seller from a small town, Otaki."
Mowbray Collectables' Major International Stamp Auction takes place on Saturday, October 14, at the West Plaza Hotel, Wellington, from 1pm.