The All Blacks centre was suspended for a dangerous tackle on Anthony Watson during the second Test against the British and Irish Lions last month.
World Rugby initially ruled that the dual-code powerhouse would not be available to face the Wallabies in Sydney on August 19, as that would serve as the last installment of his ban.
New Zealand Rugby argued that the world champions' match of two halves against Taranaki and Counties Manukau should count as one of the four "meaningful matches" he must miss.
The governing body has now ruled in favour of Williams.
A World Rugby statement said: "An independent appeal committee has upheld an appeal by New Zealand centre Sonny Bill Williams against the matches counting towards his four-week suspension for dangerous charging.
"An appeal by New Zealand centre Sonny Bill Williams against the matches which a disciplinary committee counted towards his four-week suspension following his red card for dangerous charging contrary to Law 10.4(g) during the second New Zealand versus British and Irish Lions test in Wellington on 1 July has been upheld by an appeal panel independent of World Rugby.
"A game being played by New Zealand against Counties and Taranaki on 11 August that had been excluded from the suspension by the original disciplinary committee has now been deemed to comply with the regulations definition of a "match" and, as such, that match will be included as part of the player's suspension. Accordingly, Williams is suspended from all forms of rugby up to and including 11 August."