Wellington's Lambton House building was partially evacuated after a technician found a "potentially hazardous" substance in a lift well, Fairfax reported.
Vodafone, which is among the buildings tenants, told Fairfax staff were evacuated as a precaution about 4pm yesterday after the find was made.
The company said in a statement that all staff were safe and it would divert customer calls to Vodafone's Wellington call centre to alternative locations.
Initial test results taken from common area lobbies, stairwells and offices "all returned negative results for asbestos", but the substance had been found in the lift well, Vodafone said in a further statement on Tuesday night.
WorkSafe and building owners would complete more tests from inside the lift while it was running and take dust swabs from the lift shaft.
"All testing will be undertaken with urgency, and the building owners expect results by Thursday morning at the latest."
The earthquake had been cleared of structural damage and was given a Grade A rating, Vodafone said.