Wellington police say the substance in suspicious containers found in a top IRD official's letterbox yesterday was a combination of castor oil and motor oil.
Two glass jars with lids sealed by masking tape were found in the letterbox at the Lyall Bay house belonging to Colin Hutchins, an IRD manager who led the clampdown on millions of dollars of tax evasion.
The substance was not toxic, but a Wellington police spokesman said police would be continuing their investigation into who sent the items.
Police were interested in hearing from anyone who may have information relevant to the items, he said.
Wellington police said the package had "a connection to the address" which was being investigated, but would not confirm who the owner of the property was.
Mr Hutchins is understood to have recently moved overseas and his Lyall Bay property is on the market.