Nlundulu, 23, made four appearances for the League One club last season but suffered a hamstring injury which cut short his first loan spell.
The forward developed through the Saints academy and made has played 16 times for their first team since his debut in 2020.
Nlundulu spent the first half of last season on loan at Lincoln City.
"It was frustrating for him last season because he'd only been with us for three or four weeks when he got injured and you could see that he had just got a real understanding of how we work and what we are about.
"He was really ready to take off, so I felt for him, but the beauty of it is that he is coming back to somewhere familiar, and he can pick up exactly where he left off."