Some reports say there is a possibility of it being an escape route connected to another tunnel.
Three ways out, with covers, have been found so far in the garden.
Director of Postal Services Abha Singh said that the covers had holes in them, indicating they were not conventional manholes.
“It was perhaps to allow air to pass so that people did not suffocate. If it was just for sewage then the lids would be closed completely,” he said.
Mr Rehman said that the original drawings for the building were not available in India.
Officials say they could be in the UK – the former colonial power.
“Maybe if we could access the original plan we may be able to find out if there are more – and what purpose these structures served.”