Because of the tiny Anzac beachhead, and the challenging terrain, the horsemen left their animals at Zeitoun Camp near Cairo.
Once dug in with other New Zealanders, Major Overton set off on reconnaissance missions exploring the maze of bone-dry valleys and scrubby spurs running off the high points of Chunuk Bair and a little further north, Hill 971.
It was hard, dangerous work but second nature to Major Overton, who had won his spurs as a scout in conflicts in the Transvaal and the Zululand frontier of Natal.
Moreover, it reminded him of the dry country where he raised sheep, saying it "would suit us down to the ground as it is what we are accustomed to".
The father of four's task was to find a way for troops to move up the valleys and on to the range behind the Turks, who were in trenches commanding the high ground.
In his book Gallipoli - The New Zealand Story, the military historian Chris Pugsley quotes Major Overton reflecting on the success of two missions he undertook through the Turkish lines.
"We had a most exciting and interesting time dodging Turkish outposts. I was able from what I saw of the country to make a map and gain much information as to the movements of the Turks, and would not have missed the experience for the world," wrote Major Overton.
From his observations, Major Overton made a clay map showing the topography of heavily eroded terrain.
His rendering altered the course of events in the theatre of war. British assaults further south on the peninsula at Helles had failed to break Turkish lines but Major Overton's geography suggested a way out of the stalemate. If the Allied troops could make it in numbers up the bony valleys to the high Sari Bair ridge then they could push down on Turkish positions and hammer their supply lines with artillery.
For six weeks, plans for the break-out proceeded in readiness for the assault in early August.
But many of the Anzacs were in poor shape. Sickness and disease were taking their toll.
In late June, Major Overton went to Alexandria to buy fresh food. As many as a quarter of the Canterbury regiment's 320 men were unfit for service despite remaining on duty.
Colonel William Malone, commanding officer of the Wellington Infantry Battalion, wrote: "I am not too sanguine about what we can do."
Major Overton was at the head of a column of Australian, Gurkha, Indian and New Zealand units which on August 6 began a night-time push through the dark maze of gullies. His death came in the early morning of August 7, a few days after his youngest brother Guy had died of wounds. The Sari Bair offensive, despite Major Overton's input, in the end was a failure.
Major Overton was buried where he fell, then exhumed in 1919 and moved to the 7th Field Ambulance cemetery.
His name is on the Lone Pine Memorial to the Missing.
Messages announcing the deaths of the brothers were received by their parents in Christchurch within a few hours of each other.
