A 600 metre long scrub fire that broke out on an Invercargill beach this morning has been deemed suspicious.

Firefighters were called to the blaze at Oreti Beach about 3am.

A Fenz spokesman said the fire was spreading through gorse and was about 600m long.

Conditions were windy and the response was upgraded to a second alarm.

Six pump fire trucks were on the scene, along with two water tank trucks and other support vehicles, who extinguished the fire and left the scene by 9am.

A fire investigator had visited the scene and the fire was deemed suspicious, the spokesman said.

- Allied Media