Southern, out of contract in the summer, scored his first goal since December 2012 and only his second for the club as Town secured their third win in eight league games with a 5-0 derby success against Barnsley.
The former Blackpool midfielder missed the start of the campaign with a calf injury and has made only eight appearances for Robins' side this season with competition tough for starting places in the middle of the park.
"He helped keep things ticking over in the middle of the park," said Robins after Saturday's win, in which Southern struck the opening goal in the 29th minute.
"He kept things nice and tight where we've been a little bit exposed in previous games.
"He's done particularly well here against Barnsley so I'm really pleased with his performance."