Martin Petrov's shot deflected off Campbell for number two and, although John Utaka pulled one back before Hreidarsson's sending off, Benjani grabbed a deserved third for the hosts, who remain three points behind sixth-placed Aston Villa.
"Will I still have a job? I am not worried at all. Absolutely not," he said. "It is nice people are worried for me. But you can't think about these things in life and I believe I will be sitting in this chair next season."
Pompey boss Harry Redknapp said: "We have the big four and then Tottenham and Newcastle should be behind them, so if you are outside that group and get a top-10 finish I think you've done fantastic.
"I don't know what the situation is here but they will have a top-half finish for sure and they have some quality players.
"There have been a couple of incidents this season where we have dithered and it has cost us.
"We will look at it this week and make sure we cut it out because by and large our defending has been good."