UPDATED 11.35am: Fire crews are ramping up efforts to manage the Flock Hill fire this morning and hope to be finished by the weekend.

Firefighters are on the ground today for the first time, moving in from the eight-kilometre perimeter to control hot spots.

Though the cooler temperatures and drizzle's helping, low cloud has prevented them from using helicopters and water-planes today.

DOC fire technical support Craig Alexander says they've moved into the next gruelling phase.

Department of Conservation rural fire spokesperson Andy Roberts hopes to see major progress.

"We're not sure how long it'll take before we can declare it out, but we really want the whole perimeter secured and there being no standing flames by tonight."

Roberts says six additional ground crews will be brought in today to try to finish the fire off.

"We'll continue with the aircraft, but we shouldn't be using this many aircraft and hopefully the fire will be in a much better state by the end of today."

Meanwhile, two cyclists who rode through Arthur's Pass just before a large scrub fire erupted, may hold the key to how it started.

Dash-cam footage has been released showing two cyclists and a tractor driver who police want to speak to as potential witnesses.

Detective Inspector Greg Murton says he's liasing with fire investigators.

"They're viewing the footage as well, the fire investigators, and we're just wanting these people to come through and speak to us and then we can go from there.

"It's too early to say if they're suspicious."