An independent judicial committee upheld an appeal on Thursday to allow Williams to play the Aug. 19 match in Sydney, overturning an earlier decision that had ruled him out of the clash as part of his four-game ban for a shoulder charge during the British and Irish Lions series.
A World Rugby panel had earlier ruled that the All Blacks' 'three-way' warm-up game against two provincial teams did not comply as a match and could not count as part of his suspension.
But the appeals committee said the game had sufficient meaning to be included in the ban, clearing Williams to take on the Wallabies.
Foster said the warm-up game would be played under the same rules and disciplinary procedures used in first-class matches.
"From a common sense perspective it is a game we play 80 minutes under the normal laws of rugby and we said we've done this three or four times," he told New Zealand's Radio Sport on Friday.
"It is not like a one-off or hastily organised.
"The reason we play two teams is it actually makes the game harder for us. This is not a training run."
World Rugby said in a statement that it respected the independent committee's decision but added it was "surprised by the ... interpretation of the definition of 'match'."
Foster said Glen Jackson, New Zealand's top referee, would officiate in the warm-up game.
"We treat this as a real game," he said.
"If someone gets sent off in this game they have to go through the same process Sonny had to go through.
"Under all purposes this is a match and we just wanted the chance to express that. Fortunately, we felt common sense prevailed."
(Reporting by Greg Stutchbury; Editing by Ian Ransom)
