Gas flares could be spotted out the back of Whanganui Prison near Kaitoke on Tuesday afternoon.
First Gas general manager of people and culture, Kellie O'Sullivan, said the public should not be concerned.
First Gas has a site near the prison and the flare was part of a scheduled test flare.
"It is part of a regular procedure we do to make sure all our systems are working," she said.
She said they had three scheduled flare-ups in the last six months and they carry them out throughout the country.