Otago Aids Epidemiology Group leader and study lead author Dr Sue McAllister said early diagnosis of HIV was essential for successful treatment and antiretroviral therapy.
"This treatment suppresses the virus in the body, improves the health of people living with HIV and stops transmission to sexual partners.
"If someone is diagnosed late, there is less opportunity to control progression and they can often become very unwell, requiring hospitalisation."
She said 1145 people were diagnosed with HIV during the study period, and of those, 464 (40.5%) presented late.
A quarter of those were considered to have advanced disease at the time of their HIV diagnosis.
Of all those diagnosed late, the majority were men who had sex with men. However, heterosexual men and women were proportionately more likely to be diagnosed late (55%) compared with men who had sex with men (36%).
A previous study conducted by the group, with results for the six years to 2010, showed even higher proportions of people being diagnosed late, Dr McAllister said.
"So, there is an improvement — but why there continues to be late diagnosis is not entirely clear.
"Heterosexual men and women might be less aware about the possibility of HIV and the need for testing.
"Healthcare providers may also not be thinking about HIV as a possible diagnosis when they encounter someone with symptoms, but who appears to not be at elevated risk of HIV.
"Both instances result in missed opportunities for testing and diagnosis.
"There also continues to be considerable stigma around HIV and an assumption it only affects gay and bisexual men."
While many gay and bisexual men in New Zealand were aware of the need for regular HIV testing, the paper found non-European gay and bisexual men were more likely to be diagnosed late.
"Again, why isn’t clear; but it could be harder for gay and bisexual men within certain ethnic groups to access testing services that are both non-judgemental about their sexuality and culturally safe."
She said behavioural surveys among gay and bisexual men about HIV testing, barriers to care and risk-reduction practices would shed more light on late diagnoses, but there was no easy fix.
"Comprehensive efforts towards individuals, relevant communities and the health system is needed.
"That means continued awareness campaigns, making testing easy and culturally appropriate, engaging and funding relevant groups, better funding for sexual health clinics and a focus on opportunistic screening, such as in gynaecological clinics and emergency departments."
Research by the group last year found the number of HIV diagnoses has continued to decline since the peak of 195 in 2016, and represents a 51% drop from the annual average of 137 during the previous five years (2016-20).
