One of Fa'amausili's seven siblings, Etta Jones, told the Herald the family gathered at their mother Nasareta Fa'amausili's home, in South Auckland, to cheer on the team.
"There was a lot of screaming,'' she laughed.
"The poor neighbours - I think we woke them up."
Their 73-year-old mother, a devout Catholic, watched the game in silence.
"We've never seen our mum so quiet watching a game. Every other game, mum would be screaming. But she was in total silence for most of the game until towards the end when she knew they had got it," Jones said.
"One of our sisters, she was sitting with her just holding her because we could see how emotional she was. We're Catholic and she's big on her rosary - so she was holding her rosary."
The tournament win was particularly poignant as it came on the anniversary of the death of their father, Galumalemana Tavita Fa'amausili, 16 years ago.
No one had noticed the date until their Black Fern sister, who they nickname Fi, called to remind them to take flowers to their father's grave.
After the game, the phone rang with just one request - put mum on the phone, Jones said.
"They were still at the field and Fi quickly rung. She only wanted to hear mum's voice. She just said to mum in Samoan: 'Ua taunu'u le matou taumafaiga'. (We have achieved our goal).
"Mum just cried...and told her not to party too hard,'' Jones laughed.
There were jubilant scenes from the New Zealand side at Belfast's Kingspan Stadium, as members of the team jumped and hugged each other fiercely, some crying with emotion.
They performed a stirring haka for the crowd and posed for cameras as gold streamers rained down on them.
In the excitement, the base of the World Cup trophy came off, but no one cared - it was still theirs.
Plans for a fitting celebration for the champion team, including victory parades and other welcome home events, are expected to be released in the next few days.
A spokeswoman for New Zealand Rugby said the majority of the champion team will touch down in Auckland tomorrow; with an advisory detailing upcoming celebratory events due out this week.
Wellington mayor Justin Lester was quick to offer the team a victory parade in their city, Tweeting: "Let us know when you're back home @BlackFerns - we'd love to celebrate your success with a street parade in Wellington.''
Aucklanders are yet to hear confirmation of a parade, with a spokesman for Auckland Tourism, Events and Economic Development saying discussions were yet to take place.
Talks would be made with the necessary people, including NZ Rugby, this week, he said.
NZ Rugby congratulated the team on its efforts to bring the Cup back to New Zealand soil.
Chairman Brent Impey said: "This morning's victory is the result of a lot of hard work from a lot of people and we are incredibly proud how the Black Ferns have performed throughout the campaign.
"The final was a brilliant showcase of rugby and I have no doubt the Black Ferns will have inspired young girls and boys across New Zealand with their performance.''
Impey also acknowledged the team's coach, Glenn Moore, for his exemplary work with the squad and made special mention of team captain Fa'amausili, who is now due to bow out of international rugby after five Rugby World Cup campaigns.
