"This is an opportunity for both players to play in what will be an historic game for the Cardiff Blues," said Wales backs coach Rob Howley.
"In our own squad we feel we have the strength in depth in the back row and at scrum-half to facilitate this request from the Blues to have access to their players and we have taken this decision in the spirit of cooperation with our regions and in the spirit rugby.
"Both Gareth and Sam could well have figured in our own plans on Saturday and are not entirely ruled out should we need to make any changes towards the end of the training week, but obviously they will not be named for Wales when we make our team announcement tomorrow (Tuesday).
"We will incur some disruption to our own preparations, but this decision is about putting the players first as well as backing Welsh regional rugby. This way round we will have two Welsh scrum-halves, and a young man like Sam, all with the chance to start against Australia in a week - a significant development opportunity which would otherwise not have been the case.
"Wallabies will be a tough proposition for the Blues, particularly after their set-back against Scotland at the weekend, they will be looking to get back to winning ways.
"We will be watching with keen interest and wish the Blues and Gareth and Sam all the very best as they attempt to pull off an historic result against a touring international side."