The Auckland Rugby Union (ARU) have announced new deals for 16 of their NPC squad, with current New Zealand international wing Howlett and lock Williams penning two-year deals.
Internationals Ben Atiga, Sam Tuitupou, Steve Devine, Daniel Braid, Jerome Kaino and Brad Mika have also agreed to stay.
Howlett said: "The common thread for a lot of us is that this is home, this is where a lot of our family and friends are.
"We all have experience of playing representative rugby and coming through the development system here. This is where it started for us, Auckland rugby."
Rivals Wellington have also received good news over the futures of two of their young All Blacks.
Scrum-half Piri Weepu and utility back Ma'a Nonu will remain in the capital until 2007.
Nonu had been courted by several NRL clubs after he was left out of Graham Henry's Tri-Nations squad.
Wellington rugby chief executive Malcolm Holmes told The Dominion Post: "There was a period of speculation about Ma'a's future, but we were always confident in his strong ties to the Wellington community and we are delighted that he has committed through to the end of 2007."
Lions captain and former All Black prop Joe McDonnell and Manu Samoa hooker Mahonri Schwalger will also stay put.
Holmes added: "The continued development of Piri and the experience of Joe and Mahonri will also be a key part of our mix moving forward and we were are very pleased with the quartet of re-signings."