"These databases contained information from around 8,000 clients who had contacted Family Planning NSW through our website in the past 2 1/2 years, seeking appointments or leaving feedback", the email read.
The message urged individuals concerned about the security breach to contact Family Planning NSW.
The email claimed the hackers demanded a bitcoin ransom on Anzac Day, and said the website was secured the following day.
Chairperson Sue Carrick, and CEO Ann Brassil wrote to the clients today saying the Anzac Day cyberattack "may have compromised our online databases".
Fairfax reports the personal information of up to 8,000 clients, including women who have requested advice on abortion and contraception, may have been exposed in the breach.
The hackers were able to access information entered by clients via the online form such as names, dates of birth, contact numbers and reason for their inquiries.
FPNSW said it had "secured" its website after the attack, though the site has just been a landing page since the attack that advises customers it is "getting a security update". "Thank you for your patience, we'll be back online as soon as possible".
Clients were told Family Planning NSW was one of several agencies targeted by cybercriminals who requested a bitcoin ransom on April 25.
Health service providers accounted for nearly a quarter of the breaches reported in the first six weeks of operation of the government's Notifiable Data Breach (NDB) scheme, which began on 22 February.
Australian Federal Police has been notified and FPNSW is working with the Office of the Australian Information Commissioner.
The not-for-profit has five clinics in NSW, with more than 28,000 people visiting every year.
Brassil said that the organisation was not the sole target of the attack.
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