Police are still investigating a smashed ATM and a major South Island fire that is set to major ramifications for travellers.

Fire crews spent nearly two days fighting a blaze at Porters Pass that started early Sunday and spread to more than 150 hectares of scrub.

An abandoned Nissan Mistral was found near where the fire started - which police believe was involved in tearing an ATM out of Darfield post shop's wall before the fire.

Detective Ben Rolton says they'd like to hear of anything unusual seen near Darfield or Porters Pass between two and four am Sunday, including sightings of the blue and silver Nissan Mistral.

They believe the car was stolen from a rural property in Bar

While the fire is out, there will continue to be issues for those driving through the area.

NZTA says about 2.5 kilometres of State Highway 73 at the pass will be down to one lane with stop-go signals - while a scorched guardrail is replaced and rock-falls are cleared.

Journey Manager Tresca Forrester says the fire damage has increased rockfall risk, especially with wet weather on the way.

She says it'll take several weeks before work is complete.