"This wasn't about just putting it in an envelope and sending it back. This was about making sure it got recognition and the family got recognition, and that this was reunited and given back," Mr Williams said.
"It needed to find its way back home."
The plaque - in the shape of a medallion - war medals and a scroll were issued in 1921 to Private McColl's father, who lived in Wellington, but they were returned without an explanation.
They were re-issued in 1923 and ended up at the Borough Council and being displayed during Anzac commemorations, before being sent to NZDF Archives.
The medals and the scroll disappeared, and are still missing, and it was a mystery as to how the plaque ended up at the Woodville police station, Mr Williams said.
Private McColl's nephew, Graham McColl, said the family had no idea it existed.
"Last week when Karl rang us up, I couldn't believe it."
The family had no photos of their uncle and he had no family before travelling overseas to fight in the war, Mr McColl said.
"We had no idea how this medallion came to Woodville police station, but we're very pleased that Senior Constable Williams found it and pursued inquiries to return it to our family," he said.
"It's a very emotional time for the family."
Andrew McColl said the plaque had brought his uncle "back to life" for the family.
There were 16 brothers and sisters in the family and it would be shared by all of them, he said.
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Private Andrew McColl's war career:
* 1916: joins 2nd Battalion Canterbury Regiment;
* Sept - Oct 1916: fights in the Battle of the Somme;
* Oct 1916: hospitalised with shell-shock (now known as post traumatic stress disorder);
* Nov 1916: rejoins unit;
* June 1917: takes part in the battle for Messines;
* Oct 1917: fights in the Battle of Passchedaele;
* Dec 1917: deployed to a company that did salvage work on battlefields; and
* Dec 18, 1918: died from a cerebral haemorrhage and was buried in Mons Communal Cemetery in Belgium.
