About 2000 staff have been forced to leave Woodside's headquarters after an activist allegedly used a foul-smelling substance to simulate a gas leak.
The building in Perth's CBD was evacuated about 10am on Thursday after the latest incident involving the Disrupt Burrup Hub protest group.
A woman in her 40s was taken into custody but was yet to be charged, West Australian police said.
Disrupt Burrup Hub said the woman had used ethyl mercaptan, a non-toxic substance that can be used to alert underground mine workers to emergency situations.
Woodside confirmed employees were mustered to safety outside the building as police and firefighters dealt with the incident.
Their return to the office was approved by emergency services on Thursday afternoon.
"Authorities are investigating the source of the strong-smelling fumes that prompted the evacuation," a company spokeswoman said.
"Woodside is monitoring the health and welfare of our employees and employee assistance program services have been activated.
"The safety of our people is Woodside’s highest priority.
"We respect people’s right to protest safely and legally but it is unacceptable for protest action to put our people’s safety at risk."
Disrupt Burrup Hub campaigner Kristen Morrissey was photographed holding yellow smoke flares outside the entrance to the oil and gas giant's building.
"Today, I deployed non-toxic, non-flammable 'stench gas' to shut down business as usual for Woodside and sound the alarm about the climate and cultural emergency they are enabling at the Burrup Hub," she said in a statement released by the group.
"It continues the job we started at the Woodside AGM last month, using a harmless warning to evacuate people from potential harm, in contrast to Woodside’s activities at the Burrup Hub which endangers culture, climate and their own workers."
Tactical response group officers were deployed to examine and remove an item believed to be the source of the gas.
"They are being assisted by specialists from ChemCentre," WA Police said.
"The investigation into the incident is ongoing, however at this time there is not believed to be any ongoing danger to the community."
Disrupt Burrup Hub has since been forced to move a public information event scheduled for late Thursday after the City of Perth cancelled a booking to use a council space, saying "the event posed a risk to staff, volunteers and city operations".
The group is calling for industrial development on the Burrup Peninsula, about 30km west of Karratha in the Pilbara region, to be stopped, including Woodside Energy's expansion of the Pluto gas plant.
The Burrup Peninsula, known as Murujuga to traditional owners, contains the world's largest and oldest collection of petroglyphs.
