NSW Health data relating to the Oral Health Fee for Service Scheme was lifted from a government server last Thursday.
Investigators declared the incident "major" on Wednesday."
The scheme allows the public dental service to provide patients with vouchers for work done by private dental practitioners.
eHealth's chief executive Richard Taggart said no patient data was involved in the breach, but a small number of third-party provider data may have been stolen.
Pinched data could involve sensitive documents, he said.
The body is looking to notify private dental providers who may have been affected.
NSW Health uncovered the breach after an automatic system identified unusual traffic on a remote health server.
The site was taken down manually within about three hours.
The hackers have made no demands.
The commander of NSW Police Cybercrime Squad, Detective Superintendent Matt Craft, said investigators were now turning their attention overseas.
"What we see is threat actors move data [from] within Australia offshore, so that's part of our investigation," he said.
Investigators will leverage international relationships, including with the FBI, if a suspect is identified overseas.
Unlawful access to data is a criminal offence and is punishable by imprisonment in NSW.
Detectives confirmed they were aware of online claims surrounding the breach.
In one claim, a hacker boasted that they had compromised 100,000 - or 70gb - of sensitive documents.
"We're aware of certain posts that have been made, and that will form part of our criminal investigation ... (but) we also need to test the veracity of that information as well," he said.
Online cyber security tracing service Hackmanac posted to X a screenshot of a forum post claiming responsibility.
The account alleged they could access the files before accidentally crashing a NSW Health server on the forum.
Hackmanac said its verification of the post remained "pending".
Hackers exploit datasets for ransom or leverage occasionally but they often also boast of their endeavours for credibility within the community.
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