THIS is the Big Brother house...no, the other one.

In one of the sneakiest twists in the format's history, host Sonia Kruger will trick the current crop of contestants into thinking they have to compete against a rival house of 14 other hopefuls next week.

The secret housemates, all paid actors, were filmed in a mock-up BB house on the Gold Coast, lazing about, flirting with the cameras and fiddling with their microphones in the same style of the real housemates locked inside the Dreamworld house.

The challenge will be introduced following tomorrow night's eviction on Channel 9, where Bradley, Ben and Ryan are all vying to stay in the competition.

Executive producer Alex Mavroidakis revealed the mind game had been in the planning since Christmas, with BB task producers "leaking'' details of the second house and fake contestants onto fan forums and spoiler websites.

"If we can pull it off, it will be the best week of television in the history of Big Brother,'' he said.

Only one contestant, Estelle, was given a hint of the possibility of a second house, with audio about the second house deliberately fed through her headphones as she waited to be sent on stage to meet Kruger then placed inside the real house for the first time.

Mavroidakis said housemates Michael and Ben, both with high IQs, posed the greatest risk to the trick unravelling, if their cynicism rubbed off on other housemates.

Meanwhile, intruders could be the next twist to this year's competition as producers try to keep up the drama and the ratings.