Nearly 500 cases of syphilis were recorded in 2022, up 8% from 2021 and bucking a three-year downward trend.
Dr Massimo Giola, one of the country's leading experts on sexual health, says it's not good enough.
"Over the last quarter, we have seen the biggest increase ever in New Zealand," he said.
Pacific Health chief clinical advisor Dr Corina Grey says the country has seen a rise in sexually transmitted infections "of all types, but we're particularly concerned about syphilis because of the devastating effects on pregnant women and babies".
There's even more concern for congenital syphilis cases, which is when babies are born with syphilis after the mother's been infected.
There were six cases of congenital syphilis in 2022, "which is six too many", said Giola.
"The only number of acceptable congenital syphilis cases is zero."
Making testing more readily available and accessible for pregnant women is crucial, according to sexual health doctor Frances Robbins.
"About 44% of the cases of syphilis in women is picked up during pregnancy, so that suggests there's not enough testing happening before pregnancy."
She also said the overall numbers, which are the worst yearly results since 2019, "is just the tip of the iceberg due to undertesting".
Māori rates are also disproportionate to the general population, making up more than a third of the cases last year.
The chief medical officer at the Māori Health Authority, Dr Rawiri Jensen, says the statistics are "deeply concerning".
"(In recent years) our attention has been on the pandemic somewhat more than we might otherwise have with STIs. But I think it's a call to action for us to return our attention to the importance of managing STIs, making sure we're giving patients good advice about it, how to manage it," he said.
Containing syphilis outbreaks can be difficult due to many individual cases not displaying any symptoms.
"One of the disturbing things is that most people who have it, and who are able to pass it on, will never know they have it," said Giola.
But he's quick to reassure that the disease is easily treatable if detected early.
"It's not rocket science; with a simple blood test and a simple antibiotic treatment, people can be cured."
Early symptoms of non-congenital syphilis include a sore or ulcer at the site of infection, usually the genitals, anus or mouth.
Later symptoms can include rashes, sometimes with spots on the palms of the hands and soles of the feet; swollen lymph glands; fever; headaches; tiredness; and wart-like growths in skin folds.
The Burnett Foundation's Joe Rich says sexual health services have suffered from a lack of investment for a number of years.
"Over decades we've seen the screws being put on sexual health services and frankly, their ability to respond to outbreaks like this is very limited so it's not a surprise we're seeing this increase," he said.
He says improving education around safe sex and testing will help tackle the issue, but that extra funding into sexual health services is what could be required for long-term, long-lasting improvements.
"In general in New Zealand there is a squeamishness around talking about sexual health and I do wonder if that's why we've seen the screws put on our sexual health services. It's not a sexy area of health to fund."