A large scrub fire in Nelson on Tuesday may have been caused by a spark from machinery being used to created a fire break.
Waimea principal rural fire officer Neil Eder says a grass mower may have ignited the fire.
The fire, which started in Atawhai about 3.50pm on Tuesday, moved south-east spreading across some 40 hectares of gorse and grass.
Police began evacuating about 100 people around 7pm but residents were allowed to return home three hours later as winds, which had helped spread the fire, had died down.
No damage has been reported to houses.
Mr Eder says 20 firefighters monitored the area on Tuesday night and two bulldozers were used to create firebreaks.
Eleven helicopters with monsoon buckets returned to the scene on Wednesday morning to douse remaining hot spots, to ensure the blaze does not re-ignite.
Ground crews will remain on the scene on Wednesday night.