A Mandurah mum says she is "disgusted" after a "psychopath" jumped her back fence, sniffed the underwear hanging from her clothesline, helped himself to a pack of cigarettes and left on Friday night.

At the time, Oksana Panich was having a quiet night at home watching a movie on TV with her 11-year-old son, oblivious to what was going on.

But after she noticed her packet of cigarettes was missing the next morning, she decided to review the footage on her security camera.

What she saw turned her stomach.