While standard diagnostic codes used by doctors showed about 21,000 cases from 2016 to 2020, Grant Sara and his team suspected that was a gross underestimate.

By trawling emergency department records for mentions of self-harm, suicidal, poisoning and other relevant words, and excluding cases that appeared accidental, the researchers uncovered another 30,000 presentations to NSW emergency departments.

Together, it shows at least one in 55 ED presentations are related to self-harm.

"Self-harm is one of the strongest risk factors for suicide," Dr Sara said on Thursday.

"Emergency departments are often the first point of health care for people with self-harm and suicidal thoughts."

It comes as data shows ED presentations nationwide for self-harm or suicidal thoughts have risen in the past decade.

Dr Sara said accurate data was needed to understand trends, plan services and improve the care that people receive.

"However, ED data systems are complex. In many health systems around the world, routinely collected data underestimate this important issue," the University of Sydney associate professor said.

NSW had previously needed to rely on hospital admissions, missing those sent home from ED, or other sources.

NSW Ambulance data showed they were bringing about 38,000 people each year to NSW hospitals for issues related to self-harm, but only 10,000 of those had hospital files with the standard diagnostic codes.

The enhanced method, published in peer-reviewed journal Public Health Research & Practice on Thursday, has been quietly adopted in regular NSW Health reporting.

Even still, the researchers say it still needs ongoing development and refinement.

Compared to the old technique that relied solely on specific self-harm diagnosis codes, the new method expanded the number of diagnosis codes used to identify self-harm and suicide-related behaviours.

Those included additional codes for poisoning with medications such as paracetamol, where many poisonings in adults are likely to be due to self-harm.

Text fields used by ED staff to describe the so-called presenting problem were also searched for words and phrases indicating self-harm or suicidal thoughts.

The new figures were validated against NSW Ambulance data and recent self-harm estimates from Victoria, the United States and Northern Ireland.

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Australian Associated Press