A man described as New Zealand's first serial killer has been released from prison.

Triple murderer Hayden Poulter's been in prison for the last 21 years for killing a prostitute, a massage parlour worker and the parlour's boss.

Corrections says he'll be strictly monitored for life by probation officers, and subject to 26 conditions - including not using alcohol or drugs ever again, or visiting Auckland, where the murders happened.

Non-compliance with any of the conditions can result in him being recalled to prison.

It's unclear where he's been released.