At least three probable unmarked graves were discovered using ground-penetrating radar at Cromwell Cemetery and it is very likely more will be discovered in the coming weeks as the data is analysed.

Southern Geophysical geophysicist Jack Fleming said during the mapping process his team found sites with a few surface depressions that align with unmarked graves during their time at Cromwell Cemetery yesterday.

"They show all the signatures of an unmarked burial."

The data will be analysed properly over the next few weeks when it is hoped the probable graves as well as others would be confirmed.

Mr Fleming said it was surprising to find three probable unmarked graves while out on the site.

"It’s not always the case that it shows up that clearly in what we call the raw data.

"We usually have to do a bit of processing and analysis but yeah, no, that was certainly nice."

The Christchurch-based company had offered the service for more than 15 years all over New Zealand, he said.

Mr Fleming said the mapping process involved pushing a trolley with a 350 MHz hyper-stacking antenna which was one of the leading ground-penetrating radars in the world.

Also on the trolley was a GPS antenna which allows Mr Fleming and his colleagues to correlate the data collected and get it to within 2cm accuracy in real time.

The radar collects the data, stores it and then displays it on a screen.

"Our team of geophysicists will assess the data, make our picks, we call it, and we will map out the unmarked graves or features that are indicative of unmarked graves," he said.

Otago Goldfields Heritage Trust treasurer and project manager Odette Hopgood said the work was part of the trust’s heritage sites review.

With the help of the Central Lakes Trust, Pub Charities, donations and funds raised from events, the trust was able to call in Southern Geophysical to map the cemetery’s northeast corner.

Ms Hopgood said the mapping would ensure no unmarked graves would be encroached on during repair works.