"Our victories have been really dominant and we played some outstanding rugby," said Williams.
"It was just that four and half minutes of average rugby that cost us.
"But in saying that, they're a good side [Wairarapa-Bush]. They train twice a week and had a squad of 28 players."
Going uphill in the early stages on Saturday, Wanganui eventually pulled away for the Wellington Maori, with Steelform Wanganui loose forward Tremaine Gilbert playing No8 and being the best performer on the park in the final 20 minutes, said Williams.
Winger Josaia Bogileka got a hat trick of tries, while second-five Jaye Flaws grabbed a double as part of another exceptional performance.
Fullback Tupu Tavae showed off his pace and footwork, scoring a try while adding a conversion.
The other try scorers were Gilbert, flanker Kieran Hussey and hooker Jack Yarrall, while William's first-five from the Border club James Forsythe was able to join the team to cover injuries and added two conversions.
"He works for me. He wanted to play Development but someone had to milk my cows while I was away," said Williams.
"But he had this weekend off."