Labour dropped 2 per cent to 43 per cent in the latest 1 News Colmar Brunton poll, released last night.
Asked whether the fall was because of the Government's handling of the Sroubek case, O'Connor told Newstalk ZB: "I'd say so."
But O'Connor, the MP for Ohariu and former head of the Police Association, defended Ardern's decision not to fire Lees-Galloway, who he said was "competent".
People will have their own judgments on whether the minister made a mistake by initially granting Sroubek residency, O'Connor said.
"Generally you don't fire them for making one mistake. You fire them for making the second mistake and not learning from the first one."
Ardern brushed aside O'Connor's comments, saying: "This won't be the first time I've disagreed with my MP Greg O'Connor."
The latest poll showed that National had rebounded from the Jami-Lee Ross saga to 46 per cent, up three points.
The Greens were down two to 5 per cent, New Zealand First down one to 4 per cent, and Act registered 1 per cent.
Ardern slipped as preferred Prime Minister from 42 per cent to 39 per cent.
National leader Simon Bridges was steady on 7 per cent, while his former leadership rival Judith Collins was up one point to 6 per cent.
Deputy Prime Minister and New Zealand First leader Winston Peters was steady on 4 per cent.
Translated to seats in the House, New Zealand First would be out of Parliament altogether - perhaps a motive for the parties of Government to revisit lowering the threshold to four per cent instead of five per cent.