People looking for information about abortions and contraception could have had their personal information stolen, after a major data breach at Family Planning NSW (FPNSW).

Up to 8,000 clients could be affected by the hack, which occurred more than two weeks ago.

Clients were alerted to the incident by an email signed by chairwoman of the FPNSW board, Sue Carrick, and chief executive Adjunct Professor Ann Brassil.

"These databases contained information from around 8,000 clients who had contacted Family Planning NSW through our website in the past two and a half years, seeking appointments or leaving feedback," the email read.

The email claimed the hackers demanded a $15,000 bitcoin ransom on Anzac Day, and said the website was secured the following day.

"Since the attack we have had no evidence that this information has been used by the cyber attackers," the email read.

Today, the organisation's website was down for a "security update".

FPNSW provides reproductive and sexual health services, and have five clinics around Sydney.

Adjunct Professor Brassil said the software FPNSW's website had been built on was targeted.

"I think that's important for people to understand, that this wasn't about Family Planning, this was about a hack to software," she said.

"We are really sorry that has happened and we take responsibility for that.

"It was one of a number of cyber attacks on that particular piece of software on or around the same time and the ransomware note was specifically for bitcoin."

Adjunct Professor Brassil said refusing to pay the ransom was the only interaction FPNSW had with the hackers.

"It wasn't that sophisticated, in that they were only after the money and once we didn't pay the money they disappeared," she said.