Late last week the board wrote to 3239 Dunedin Hospital patients and 1379 Southland Hospital patients who are overdue for an appointment.
In the letter the board apologises and says the backlog is being cleared.
The SDHB has gone public about the issue ahead of the release, in the coming weeks, of the New Zealand DHBs' 2015-16 serious adverse events report, in which the cluster has to be disclosed.
Interim SDHB chief executive Chris Fleming told the Otago Daily Times more cases of harm could be identified as the backlog was cleared, particularly at Dunedin Hospital.
"We are on top of it in Southland. The numbers in Dunedin at this stage are still rising,'' he said.
An external investigation team had been engaged to investigate the cases.
Of the 30 patients who suffered harm due to the delays, 21 were in Southland, and nine were in Dunedin.
In Southland, an earlier cluster of four patients who suffered sight loss was disclosed in last year's serious adverse events report. In response to Otago Daily Times queries, the board disclosed another two cases, making a total of six at that time. It said extra staff had been added in Southland, as well as other measures.
The ODT featured the story of Gore man Koby Brown, who lost the sight in one eye and said he had been told by Southland Hospital staff to be "patient'' about his overdue appointment.
Mr Fleming admitted yesterday the board should have done more at that time.
"To be blunt, we didn't do enough [then].
"Clearly, the numbers speak for the results.
"We're putting this out there publicly now to make sure that we are letting all those 4600 patients know, and that then we are both publicly and privately held to account to make sure we clear this problem.
"We have a plan in place in Invercargill because the issue is more known there, and we're still developing a detailed plan [in Dunedin].''
Mr Fleming had requested an extra ophthalmologist and optometrist be recruited immediately in Dunedin.
In Dunedin, 37% of all ophthalmology appointments are overdue.
In an interview yesterday, Mr Fleming and chief medical officer Dr Nigel Millar emphasised the high demand for a relatively new treatment for macular degeneration called Avastin.
Dr Millar said the service had become "overwhelmed'' because of the success of that treatment.
"It doesn't just affect the patients receiving Avastin, it puts demand on the whole service,'' Dr Millar said.
Avastin demand was cited by the board last year to explain the earlier cluster.
"I think you've heard what [Mr Fleming] said that we can see now we didn't do enough,'' Dr Millar said.
"People believed at the time that it would be enough.
"This is something we sincerely regret and want to apologise to any of patients who suffered avoidable harm.''
The board is looking at new ways of managing some ophthalmology patients, which would involve GPs and nurses.
"We're doing some of that [already]. We've got some nurses now doing follow-up for patients with glaucoma,'' Dr Millar said.
The letter to patients, obtained by the ODT, tells the patient they are among a large number whose appointments are delayed.
The backlog would be cleared over time, but patients with immediate concern about their vision would be seen urgently.
Affected patients could call 0800 866 000 or contact their GP. They could also call hospital nurses in Dunedin or Invercargill.
Southern GPs were told in an email yesterday about the situation. Cromwell GP Dr Greg White told the ODT he did not know why the DHB had done that.
"It's been a long-term problem, so we're not sure why they have notified us again about them,'' Dr White said.
Are you affected? Contact the ODT newsroom on (03) 479-3574.
