She said the play "packs one hell of an emotional punch" -- mainly due to the commitment of her cast.
"They had me and three grown men in tears at rehearsal the other day -- and that takes good acting," Mrs Bushell said.
"These guys are digging deep.
"They're getting to grips with their characters and the emotional toll of war."
Once on Chunuk Bair follows the Wellington Battalion troops leading up to the battle, exploring the relationships between the soldiers and their superiors.
One of the key characters is Captain Connolly, based on Lieutenant-Colonel William Malone, who famously defied British orders to attack Chunuk Bair by daylight.
To tap into the military mentality, Mrs Bushell had her cast learn some army drills, be trained in handling weapons, and do team-building exercises -- with those playing the officers assuming firm leadership.
"The military discipline was to obey without question," Mrs Bushell said. "So we did some activities with the officers in charge so they could step into that hierarchy."
Before rehearsals started, the actors pored over soldiers' letters, diaries and other historical accounts, and visited the Gallipoli exhibition at Te Papa.
Mrs Bushell said their research has helped contextualise the concepts explored in the play, such as the mechanised face of war, the Maori soldiers' involvement, the Turkish perspective, and the effects of shellshock.
She has also encouraged the cast to call to mind the grisly conditions which greeted the Anzacs on the Gallipoli peninsula.
"They've read accounts from soldiers which described four distinct smells: urine, sweat, cordite explosives and rotting flesh.
"The men had dysentery, lice and fleas, had little water, and were out there in baking heat.
"A lot of them used dark humour to cope with the horrors around them, as we see in the play."
While the play has been a sobering experience at times, Mrs Bushell said been "a joy" to see the cast come to appreciate New Zealand's history.
"They have realised the gravity of what their characters went through, and they've grown as actors.
"They understand the importance of remembering these young men -- the sons, brothers, husbands and lovers, who were wiped out."
Once on Chunuk Bair opens on Saturday August 8 at 8pm, and runs until Saturday August 15. Tickets are available from www.eventfinda.co.nz.