The usernames, passwords and PINs were posted to Pastebin on Thursday, along with the banner 'OpFreeAssange' and a quote from Wikileaks founder Julian Assange. The hacker, '0x00x00', stressed that he or she was not a member of Anonymous.
In a statement, Hertfordshire Constabulary said it was investigating the breach, and confirmed that the information had been "stored on a database linked to the public Safer Neighbourhoods pages of the external Constabulary website".
The force said the database was externally hosted. The Pastebin document refers to Amazon Web Services, but a Hertfordshire Constabulary spokesperson was not able to confirm AWS was the hosting provider.
Hertfordshire Constabulary said it had disabled part of its website as a precaution, but there was "absolutely no suggestion that any personal data relating to officers or members of the public has been, or could have been compromised".
A spokesperson for Nottinghamshire Constabulary said in a statement that, following initial inquiries, the force was "confident at this stage that no restricted or confidential information was accessed or databases compromised".
"We continue to investigate and have taken steps to further strengthen security around our website as a precaution," the statement continued.
The hacker said in the Pastebin document: "This is nothing big not some l33t h4x shit ... but this tells how insecure the Web is."
Julian Assange is currently holed up in the Ecuadorian embassy in London, having claimed asylum there after losing his fight against extradition to Sweden, to face questioning over sex crime allegations.
Assange maintains that, if sent to Sweden, he will then be deported again to the US to face charges over US military and diplomatic secrets that were published through Wikileaks.
His situation has sparked several online attacks, against targets such as the Ministry of Justice, that have apparently been perpetrated by members of the hacking collective Anonymous.