He snaps photographs like an excited tourist as Herbert's Children of Israel come into view.

Measuring 3½ by 6½ metres, Moses is epic in its scale of ambition and, for the gallery, for the scale of conservation complexity. Painted on paper glued paper-mache-style to canvas, and stored since the middle of last century on a huge log roughly studded with half-hammered nails, the painting has been through a lot.

Herbert's epic comes out first in a muddy brown, with dull blues and reds slowly resolving themselves into Israelites as more of the painting is revealed. Moses, bearded, towers above all.

Mr Varcoe-Cocks has seen it unrolled once before, but even so he's a little nervous, though not as nervous as the green-gloved volunteers, who tremble slightly as they slowly unroll the canvas onto the gallery's floor.

He lets out a little sigh as the canvas comes to rest, its corners still slightly curling. "It's held up surprisingly well, to be honest," he says.

"Its scale and ambition of composition - the complexity of the figures, the rendering of all the detail and the costumes. Herbert travelled to the Middle East and did sketches of the landscape, tried to capture the light and the colour as much as possible. it's not just a studio depiction - it's a scholarly attempt to recreate a sense of the moment."

In full public view, it will spend the next few months having its surface-dirt cleaned - part of the reason why the painting's colours currently look so muted - and its canvas repaired and strengthened, before finally being hoisted onto a wall.

When the canvas is fully unrolled, the wooden reel on which it was transported via wooden ship from England can itself can be inspected. It's gnarled and scarred, both with the knots of the tree that it came from and black nails that have been half-hammered into its surface.

Glancing back at the painting one can see the spots where contact with the reel - and with the canvas itself, as it was wrongly rolled inward rather than outward when it was first stored - has stripped paint and left punctures.

Moses is actually a copy of the same work the artist did for the House of Lords, in London, which says something about the thinking of the men in charge of the gallery at the time of the painting's acquisition, as does the mid-century decision to send the work to storage.

"There are actually two other versions, the other scaled version of this in Westminster deteriorated quite badly and doesn't look great at all, so this is the best colouring and best preserved depiction that exists," Mr Varcoe-Cocks says.

When he heard in 1872 that the NGV wanted a copy, Herbert painted with oil on top of the original sketch he'd done to plan the first work. The paper was then glued onto a canvas for hanging, before being wrapped up and shipped to the Antipodes.

Sandi Mitchell, the project co-ordinator, scuttled the length of the painting as it was unwrapped, ensuring the tension applied was just so. She points out the battered brown edges of the canvas, which look like rotted paper mache. The conservation team does not even know how many layers of paper Herbert glued together, but they are hoping to find out.