People wanting to avoid chlamydia were advised to practice safe sex and be tested for the sexually transmitted infection.

One in three women and one in five men have had chlamydia by the age of 38, a study estimates.

The high rate of the sexually transmitted infection (STI) has prompted a plea for more Kiwis to get tested.

The latest findings from the Dunedin Multidisciplinary Health and Development Study, known as "The Dunedin Study", show chlamydia is more common than previously thought.

The rate of infection could be higher for younger people, who were born more recently than the study group, lead author Dr Antoinette Righarts, of the Department of Preventive and Social Medicine at the Dunedin School of Medicine, said.

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She said STIs "hit this wall when HIV came out and people's behaviour changed over a remarkably short time, but later the safe sex messages started wearing off".

Infections occurred when the study's cohort were teenagers and young adults, but was before New Zealand – and other high-income countries – experienced a marked increase in chlamydia in the late 1990s, she said.

At the age of 45 most women in the cohort had completed their reproductive lives, but concerns remain for those aged in their 20s and 30s.

Dr Righarts said the safest thing for people entering a sexual relationship was to use protection, such as condoms, and have a chlamydia check-up.

"It is not good enough for one person to do the test, both have to do it.

"Get tested and treated. It is never too late to get treated and not pass it on anymore."

United Kingdom estimates indicated 17 per cent of infections in women progressed to pelvic inflammatory disease, with 0.5 per cent of women becoming infertile and 0.2 per cent having an ectopic pregnancy due to irreversible fallopian tube damage.