The Otago Corrections Facility (OCF) was one of three prisons nationwide in the ''needs improvement'' category of the system last year, and it remained in the lowest rank until March.
It has since moved up two levels to the ''exceeding'' category in the latest ratings for the year to June.
Only one jail, Waikeria, has gained more ground, moving up three levels from the worst to the best category this year.
The OCF has improved its internal procedures which were previously not up to standard, it has exceeded the standard for rehabilitation and has had no security breaches.
Criticism of the Milburn jail has been voiced this week, relating to ongoing police investigations into the deaths of two remand prisoners, in 2010 and 2011.
The jail's health service has come under scrutiny from relatives of the deceased and a drug and alcohol counsellor.
A Corrections spokeswoman yesterday said the OCF was one of 11 prisons in which health services had achieved Cornerstone accreditation - the Royal New Zealand College of General Practitioners (RNZCGP) standard for general practice.
''This is a difficult process with very high expectations from the RNZCGP. Cornerstone accreditation means the prison health service has met the same standard applied to general practice in the community,'' she said.
Other prisons to achieve Cornerstone were Rimutaka, Hawkes Bay, Spring Hill, Auckland Women's, Tongariro, Waikeria, Christchurch Women's, Christchurch Men's, Rolleston and Wanganui.
More prisons were working towards the accreditation, the spokeswoman said.
She said prisoners underwent checks and assessments for their mental and physical health needs while in custody.
''These include mental health screening, drug and alcohol screening, and assessments to check if they are at risk of self-harm or suicide. Every death in custody is a tragedy and Corrections has been working to reduce the suicide rate,'' she said.
Southern police have been reviewing the investigation into the death of Richard John Barriball (42), who took his own life while on remand at the OCF in October 2010.
His sister has publicly voiced her concerns about the care he received while in jail, and a coroner found his level of care was sub-optimal.
The Corrections spokeswoman said frontline prison staff received mental health training and a mental health screening tool was being introduced.
Corrections was improving the referral process for forensic care and providing a more therapeutic environment in at-risk units, she said.
''The department's processes ensure many prisoners are prevented from self-harming or attempting suicide.''
Corrections acknowledged the professionalism of staff working in prison health services with some of society's most difficult and challenging people, she said.
''Many prisoners have very high health needs and have often not accessed the health care they need in the community. They may be suffering extremely poor health (both physical and mental) by the time they arrive in prison.''
A former OCF prisoner complained about the medical treatment he received in jail for a touch rugby injury, Radio New Zealand reported yesterday.
