Research suggests a fifth of gay and bisexual men with HIV in Auckland don't know they have the virus.
It's the first time a measure of undiagnosed HIV has been recorded in New Zealand.
The Otago University study suggests 6.5% of gay and bisexual men in the city are infected with HIV.
The study's head, Peter Saxton, says 20% of that group are not aware they are carrying the virus.
Dr Saxton says people with undiagnosed HIV are delaying treatment that can improve their quality of life and life expectancy.
He says the study is timely as, in 2010, the highest number of new HIV diagnoses were recorded among gay and bisexual men.
Dr Saxton says that indicates HIV remains a major health problem for New Zealand.
The study involved 1049 men who completed an anonymous questionaire and supplied a saliva sample.