The Giants struggled due to foul trouble, with star import McKenzie Moore only able to play 24 minutes, while fellow import guard DeRonn Scott fouled out.
As a result, the Sharks pulled away in the fourth, holding a listless Giants offence to just 11 points to take the 15 point victory.
Australian import Todd Blanchfield had 30 points and seven rebounds for Southland after a run of relatively quiet outings. Tai Wesley had eight points, 13 rebounds and five assists, while Duane Bailey added nine points and 16 rebounds.
Kevin Braswell - who will retire at the end of the season - had 16 points but shot 4/17 from the field.
For Nelson, Mika Vukona had 12 points and 10 rebounds, while Moore added 14 points and seven rebounds as the Giants were left to rue shooting a woeful 7/33 from beyond the arc.
Southland are now 14-3 on the season with one game remaining, while the Giants have dropped to 12-5.
The loss puts the Giants behind the Wellington Saints, who moved to 12-4 with a 97-71 win over the now 0-16 Taranaki Mountain Airs.
The scheduled game between the Hawke's Bay Hawks and the Canterbury Rams did not go ahead due to the Rams being stranded in Wanganui due to flooding.