Soldiers said fighting through a maze of rubble, in appalling weather, was "absolute hell".
Cassino fell in May 1944 to British and Polish troops, with support from New Zealand artillery.
The 28 (Māori) Battalion played a crucial part in the Battles of Cassino – it was the only New Zealand battalion able to cross the flooded Rapido River and attack the town's well-defended railway station.
Soldiers seized positions in and around the station but were forced to withdraw when German troops counterattacked.
More than 150 of the 28 (Māori) Battalion soldiers were killed, wounded or captured in the battle for the station.
Ruawai, who served from 1943-45, received head wounds and spent the rest of the war in a German prisoner-of-war camp.
"He was well known within our Tūhoe iwi as a strong advocate for veterans when he returned," Andrew said.
"He was always pushing to get compensation for veterans, and dedicated himself to helping them."
Ruawai and wife Riria were passionate about their culture and raised their six tamariki and 28 mokopuna to be fluent Māori speakers – something Andrew considered a blessing.
Andrew grew up as one of eight children of Sue and the late Nga Andrew in Kawerau. Spurred on from a dare he left Kawerau College to join the New Zealand Army 35 years ago.
That dare turned into a career that has taken him around the world.
Andrew has served in Afghanistan, East Timor and the Sinai. He has been to commemorations at El Alamein in Egypt, Beersheba in the Middle East and to Gallipoli twice – including the 100th anniversary in 2015.
This week at Cassino, tracing the footsteps of his beloved grandfather, has been something extra special for Andrew.
"I was really curious to see the area where they fought. I had visualised where he might have been when he got hit and how it might have played out when he was captured, what he was going through," he said.
"I'm honoured to be on this trip, and to have my grandfather's spirit with me. He would approve and he would be very proud of my role here at the commemorations. I was very close to my grandfather."