Armed with a fluxgate gradiometer and a hand-held GPS unit, Dr Hans-Dieter Baderwalked over more than 25sq km of cemetery ground around Central Otago in May last year.
The machine measured magnetic variations in the earth that indicated where soil had been disturbed and from that, Dr Bader, of Auckland-based Archaeological Solutions Ltd, identified possible unmarked graves.
Maps created during the geophysical survey were laid over aerial photographs of the cemeteries, in collaboration with Otago University's Southern Pacific Archaeological Research, to provide a map of all graves, marked and unmarked.
His report, released this week said most of the unmarked graves were likely to date from before 1900.
In the Drybread cemetery, for example, the potential graves identified in the survey were found to be in a different alignment from the graves shown in the cemetery plan, which led him to believe they were older than the marked graves.
In some cases, original cemetery records had been destroyed and others were old and particularly fragile. Others still were known to be incomplete records.
''The problem of unmarked graves is a common one in cemeteries located in settler or first-generation landscapes.
''These landscapes evolved when the ethnic, cultural and religious foundations of New Zealand society were becoming established but this was also a period of mobility, impermanence and change. Unfortunately, this was also a period when record-keeping was imperfect and the pace of change rapid,'' he said in his report.
Not all ground in each of the 15 cemeteries was covered, in most cases, Dr Bader was directed where to go by administrators of the cemetery.
A 2010 Central Otago District Council-produced document identified the majority of cemeteries in the district had areas of unmarked grave sites.
A report by the council's Maniototo property and facilities officer, Janice Remnant, said that created operational issues as large areas in those cemeteries were unable to be used for burials.
Those issues, as well as ''vulnerable'' maps and a want for digital information instigated the survey, she said.
Dr Bader's report is set to be presented to the district's community boards at upcoming meetings, along with Ms Remnant's report.
It will be up to the administrators of each cemetery to decide what to do with the information.
''The cemeteries of Central Otago contain a fascinating record of our rich and diverse past and this research is a step in the right direction towards revealing that past,'' Dr Bader said.
Eight cemeteries in the district are controlled by trusts which together funded $5550 of the project. The Cromwell, Vincent and Maniototo community boards each gave $6000 and the Roxburgh board gave $2000. The Central Lakes Trust granted $18,335.
The project came in under budget by about $9000 because various components, such as excavation work, were not needed.
Unmarked graves
Number of possible unmarked graves in Central Otago by cemetery:
Blacks (Omakau) 40
Blackstone (Oturehua) 14
Clyde 138
Cromwell 64 (53 in ''Chinese section'')
Drybread 54
Ettrick 28
Graveyard Gully (Alexandra) 14
Kyeburn Diggings 70
Millers Flat 90
Naseby 93
Nevis 29
Moa Creek 55
Roxburgh 65
St Bathans 4
St Bathans (Catholic) 20
