They would add burial spaces that could be used and reused for centuries to cater for NSW's growing population.

Chief executive of the Eastern Suburbs Memorial Park Graham Boyd confirmed last week that the cemetery had been investigating the feasibility of building Australia's first catacombs.

The early-stage concept proposes a structure resembling a multi-storey carpark, which would provide as many as 4000 graves providing double internments.

"The upper level is pristine," said Mr Boyd of the early plans. "It will be architecturally pleasing with natural light, while the lower levels are moving to a catacomb in a Roman style."

The roof of the area will resemble a standard graveyard, covered in lawn.

"If it is engineered, you could have three burials on one level, with potentially 7000 interments," Mr Boyd said. "This could create a model for centuries.

"The ossuary voids could be used by families again and again in perpetuity, and you can go on until the voids are filled. That could take centuries," he said.

He said this process would "virtually perpetuate the use of the surface crypt for the family or group who possesses the burial rights for it".

Mr Boyd said it was also investigating the use of ossuary boxes at Woronora Cemetery - which is part of the Eastern Suburbs Trust management.

This would involve deepening older graves and moving remains into ossuary boxes, freeing the grave to be used by another family member. This proposal has been discussed with the Greek Orthodox Church: about 70 per cent of its dead are buried in the Sydney region.

Catacombs have been used for thousands of years. The Christians developed the technique and created a "vast and multi-levelled network of galleries", says a history of the Early Christians.

"This was the solution to the problems of burial of a large community with an ever-increasing number of members," it said.

Nearly 70 per cent of the dead are cremated in NSW, yet groups like the Greek Orthodox, Muslims, Jews and others bury their dead, putting pressure on cemeteries to provide religiously appropriate burial sites.

The proposal would extend the life of the Eastern Suburbs Cemetery, which only has another 15 years of cemetery space left.

The NSW state government is currently conducting an audit into burial space.

By 2051 the number of deaths every year is expected to double to more than 300,000.

News of the catacombs idea - under consideration for some time - coincides with Rookwood Cemetery's plan to bury 100 pigs to see if decomposition can be hastened so graves can be reused more frequently.

Because of the rising cost of burials and a shortage of space in Sydney, the use of ossuary boxes was a good alternative for churches that did not cremate, said a Greek Orthodox priest.

Father Constantine Varipatis, of the St Stylianos Parish, a Greek Orthodox church in Gymea near Woronora cemetery, said ossuary boxes were common in large Greek cities like Athens where burial space was scarce.

"After three years or so, they dig up the body, wash the bones in a special way (with water and or wine) and put them in an ossuary box," he said.

Burial was special to Greek Orthodox Christians: "We believe as Christians that that there will be a resurrection of body and soul, that's why we don't cremate, and we leave bones in the earth.

"Greek Orthodox also believe the bones of saints - holy relics - give off a sweet smell which lets followers know that they have been transformed into saints by God's grace."

Father Constantine had been involved in preliminary discussions about the introduction of ossuary boxes at Woronora Cemetery, but stressed the proposal would need approval from the NSW health department and the diocese.