Their home in Whakamarama became a treasure house of exotic figurines and objects, Thai furniture, Persian rugs and more than 300 paintings and sculptures.
Following Theo's death, Koula continued to add to the collections and became a well-known figure on the local art scene.
She attended numerous local exhibitions and travelled to Rotorua, Hamilton and Auckland for inspiration and to acquire new works.
The Koula exhibition is only a selection of the couple's impressive collection. It represents works by local, national and international artists alongside Koula's own studio and paintings.
"The school holiday programme inspired by the Koula exhibition offers separate sessions for different age groups, to make it as enjoyable as possible for all participants," says the gallery's learning and engagement programmes manager, Hannah Wilson.
"Parents or caregivers can go to Tauranga Art Gallery's new website [artgallery.org.nz/events] to make bookings." The cost per session is $20, which includes all materials used.
Hannah says it would be a good idea to book in early as the holiday programmes usually sell out. She also advises to make sure children come dressed in 'art' clothes, as it could get a little messy.
"With the new online booking system, we can no longer offer the deal of any two classes for $35," she says.
"Instead, there is a TAG art studio loyalty club cards which gives a sixth TAG art studio visit for free. This is a slightly better deal than before," Hannah explains.
The cards will be kept at the gallery's reception area and stamped when a child is signed in. An adult is required to sign children in and out of the session.
See artgallery.org.nz for more information and to make a booking.
The exhibition Koula: the Collections of Niki and Theo Vellinga can be seen until March 12.