"It's designed really to prevent accidental tripping" overboard, she told reporters.
"We want to make sure that it's not possible for people to fall overboard or to trip and fall overboard ... so I think it would be highly unlikely, but again, in this case, the police are conducting a full investigation," she said.
The ship has around 600 surveillance cameras that are constantly monitored, although no one reported seeing the fall at the time. Sherry defended the level of monitoring of passengers aboard the ship that allowed the couple's fall to go unnoticed. At least four crew were monitoring the ship's surveillance cameras at any time, she said.
The night Rossington and Schroder vanished, the video surveillance staff was busy watching the public areas of the ship, she said.
"It was the last night of a cruise," Sherry said. "Virtually everybody else was in the public spaces on the ship, and they're the areas that we focus on in those times."
The couple were among 2,680 passengers on a South Pacific cruise. The ship's last stop was Mare Island in New Caledonia, which it left on Monday, bound for Sydney.
The emergency is the latest high-profile problem for Miami-based Carnival Corp., the world's largest cruise operator.
Last year, the Costa Concordia ran aground off the coast of Italy, killing 32 people. Also last year, the Costa Allegra caught fire and lost power in the Indian Ocean, leaving passengers without working toilets, running water or air conditioning for three days. Costa is a division of Carnival Corp.
In February, passengers aboard the Carnival Triumph spent five days without power in the Gulf of Mexico after an engine-room fire disabled the vessel. Those on board complained of squalid conditions, including overflowing toilets and food shortages.
Associated Press writer Rod McGuirk contributed to this report from Canberra.