He also set up one of his team's three tries and scored a brilliant eight-pointer of his own after regathering his chip kick from Buller's own 10m line.
Cameron Hayton scored two tries for the Bush as they led at halftime, and Andy Humberstone made one successful penalty to tie the match after Lash's try, but could not add another shortly afterwards.
After early losses to Buller and Wairarapa Bush, Mid Canterbury have ultimately climbed up into the Meads Cup group and will look to consolidate their spot with a home game against Thames Valley this weekend.
They scored ten tries in Ruatoria, with Maleli Sau and Jordan Stewart both collecting doubles.
The Cantabrians have been assisted by the extraordinary game in Te Kuiti as King Country and North Otago, neighbours on the table, fought out a tryless 4-4 draw in heavy rain conditions.
Both teams scored two penalties, with North Otago's Tyrone Davies adding his second in the dying moments to keep the teams firmly ensconced in the middle of the Lochore Cup group with two rounds to play after one of the lowest scoring NPC games in recent memory.
There must have been quite the breeze at Greymouth's Rugby Park, as Poverty Bay would move up into the Lochore group by virtue of a 62-28 win over West Coast.
Poverty Bay scored 46 unanswered points in the second half after they were trailing the Coasters 28-16 at the break, with powerful prop Jody Allen scoring a hat trick of tries in his second match for the province.
Ratu Daurua got a double, while Isaia Vuki scored a try and added seven conversions.
While their upcoming derby game with North Otago is never a sure thing, South Canterbury will await the result of Wanganui vs Buller with great interest, as they remain two points behind the leaders after their 36-24 win over a competitive Horowhenua-Kapiti in Timaru.
Leatigaga Vini scored two of South Canterbury's five tries, as they had to work hard in the second stanza after being deadlocked 16-16 at halftime.
Results, October 1
Poverty Bay 62 (Jody Allen 3, Ratu Daurua 2, Isaia Vuki, Murray Hewson, , Kieran Short tries; Vuki 7 con) bt West Coast 28 (Chad Kingi-Ternant, Amenatave Tukana, Regan Stanton, Troy Tauwhare tries; Nik Cumming 2 con). HT: 28-16 West Coast.
Mid Canterbury 76 (Maleli Sau 2, Jordan Stewart 2, Jarred Percival, Christian Vainerere, Mike Lynch, Andrew Letham, Seta Koroitamana, Timoci Nabakeke tries; Percival 5 con; Leatham 3 con) but East Coast 22 (Caleb Lewis, Tavita Ahoafi, Long Fisiihoi tries; Sam Parkes 2 con). HT: 38-6.
South Canterbury 36 (Leatigaga Vini 2, Zac Southen, John Dyer, Miles Medlicott tries; Southen 2 con; Willie Wright con) bt Horowhenua Kapiti 24 (Ryan Shelford, Aaron Lahmert, Scott Cameron tries; Warwick Lahmert 2 con, pen). HT: 16-16.
Buller 26 (Phil Beveridge, Setefano Sauqaqa, James Lash tries; Lash pen, 3 con) bt Wairarapa Bush 24 (Cameron Hayton 2, Sam Gammie tries; Tim Priest 2 con; Andy Humberstone pen). HT: 22-16 Wairarapa Bush.
King Country 4 (Zayn Tipping 2 pen) drew with North Otago 4 (Tyrone Davies 2 pen). HT: 2-2.
Wanganui 60 (Jamie Hughes 2, Kameli Kuruyabaki 2, Te Rangatira Waitokia, Tremaine Gilbert, Gavin Thornbury, Michael Nabuliwaqe tries; Dane Whale 5 con; Te Tau Kemp con) Thames Valley 14 (Hayden Wishnewski, Haere Renata tries; Beau Hamilton con). HT: 52-8.