The efforts saw the Kiwis finish on 12 points to New South Wales six.
Riders from both teams rode borrowed horses, swapping throughout the competition.
"Both teams were pretty even really," said Howells. "It comes down to a bit of luck in the end but we were more consistent come the second round of each day - that certainly is where we were at our best."
The New Zealand team was selected after expressions of interest were called, and was trained by Merran Hain of Gisborne.
Meanwhile Hawke's Bay horses and riders took the lion's share of the New Zealand young horse titles with Ocean Beach stablemates Bridget Hansen and Dee taking out the seven and six year old championships respectively on Toulon NZPH and Unlimited NZPH, with Kim Best and Duke winning the four year old championship.
Katie McVean of Mystery Creek and Casebrooke Lomond won the five year old championship.
Results:
Trans Tasman Challenge: New Zealand 12 points, New South Wales 6.
Outpost Buildings/Reins on Ormond Seven Year Old Championship: Bridget Hansen (Ocean Beach) Toulou NZPH 1, Joy McNeill (Kamo) Bella Vi 2, Kim Best (Hawke's Bay) Mr Wiggle 3, Molly Bennett (Wairoa) Landsman GNZ, James Cole (Auckland) Blue Lagoon, Katie McVean (Mystery Creek) Dunstan Carlo, Katie McVean (Mystery Creek) Dunstan Lucca, Jesse Linton (Hawke's Bay) AP Ninja, Bridget Hansen (Ocean Beach) Turn it Blue NZPH =4.
Mitavite Munga/Extreme Sporthorses Six Year Old Championship: Luke Dee (Ocean Beach) Unlimited NZPH 1, Vicki Wilson (Hikurangi) Lanstar Xtreme 2, Sally Clark (Dannevirke) Wishing 3, Maurice Beatson (Dannevirke) Mandalay Cove, Katharine Gillaly (Palmerston North) Double J Sunshine, Luke Dee (Ocean Beach) Ulysses NZPH, Antoine Ogilvie (Pukekohe) Ripple Effect =4.
Canterbury Equestrian/Farmlands Five Year Old Championship: Katie McVean (Mystery Creek) Casebrooke Lomond 1, Lucy Olphert (Tauranga) Eve Saint Laurent 2, Amanda Wilson (Hikurangi) Showtym Rebel 3, Tinesha Carey (Hawke's Bay) Double J Monarch 4, Caroline Kennedy (Cambridge) Smoken Up 5, Luke Dee (Ocean Beach) Velvet NZPH 6.
Kelso Four Year Old Championship: Kim Best (Hawke's Bay) Duke 1, Amanda Wilson (Hikurangi) Woodbine Legacy St 2, Tinesha Carey (Hawke's Bay) Quinella, Tinesha Carey (Hawke's Bay) Double J Whitney =4.
STH Hay and LG Forge/Hawke's Bay A and P Society Pro Am Rider: Jamie Howie (Taihape) Otairi Olivia 1, Jamie Howie (Taihape) Zalula 2, Christine Cornege (Waikato) Varekai SL 3, Laura Knight (Palmerston North) Sir Arthur 4, Samantha Peters (Whangarei) Zabambi 5.
Telford/Northern Hawke's Bay Junior Club Young Rider: Lily Tootill (Karaka) Ngahiwi Cruise 1, Luke Dee (Ocean Beach) Ombudsman 2, Bridget Hansen (Ocean Beach) Shakespear NZPH 3, James Cole (Auckland) Chequebook 4, Rose Alfeld (Christchurch) My Super Nova 5, Melody Matheson (Hawke's Bay) Cheltenham 6.
PGG Wrightson Open Horse 1.4m: Katie McVean (Mystery Creek) Dunstan Breeze 1, Claire Wilson (Waipukurau) Tipsey 2, Vicki Wilson (Hikurangi) Glenara Chandon 3.
KiwiSpan/Ngamatea Junior Rider: Caroline Kennedy (Cambridge) Classic Arrow 1, Georgia Massie (Dannevirke) Kiwi Ironheart 2, Lucy Bull (Taupo) Cheleken Cosmo 3, Lucy Allen (Morrinsville) Delta Pierre 4, Melanie Weal (Te Awamutu) Cross Your Heart 5, Lily Tootill (Karaka) Yum Yum 6.
Caledonian/Silver Fern Farms Amateur Rider: Graeme Isaacson (Waipukurau) Lykon 1, Joanne Mackie (Gisborne) X Factor MVNZ 2, Diana Cottle (Levin) Morpheus Rising 3, Helen Bruce (Palmerston North) Toblerone NZPH 4, Merran Hain (Gisborne) Untouchable 5, Kaylene Dudding-Thomas (Napier) Hugo II 6.