Michael Cheika's Wallabies wrap up 2018 with November tests against Italy, Wales and England, having lost seven of 10 matches this year to slump to seventh in the world rankings.
"They are determined to go there and win all three games," Castle told reporters in Sydney on Thursday.
"That's the start point, that's the pass mark from where they are, from their perspective, that's the expectation that certainly Michael's setting for them and there's no reason why they can't do that.
"They will be very confident that they can come home with three wins."
The Wallabies' struggles have triggered calls for Cheika's sacking by prominent media pundits but RA has steadfastly backed the 51-year-old to turn the ship around before the World Cup in Japan in a year's time.
Australia were thrashed 37-20 by the All Blacks in Yokohama on Saturday to be whitewashed 3-0 in the annual Bledisloe Cup series contested between the nations, but New Zealander Castle saw positives in the most recent loss in Japan.
"I think the great thing about the third test was we saw some improvements and some things they identified they were concerned with," she said.
"I'm not justifying the result -- we all wanted a win -- but if you think about building into a World Cup, there was some very specific improvements, particularly around the defensive patterns and things I know they had targeted, that Michael had (targeted), as important for that game."
Castle was speaking to reporters after announcing a tie-up with the Australian Army to share facilities and intellectual capital in high performance, injury management and leadership.
The partnership would likely see Wallabies players joining the nation's special forces soldiers in programmes to improve performance under pressure, according to Brigadier Ben James, the president of the Australian Army Rugby Union.
"So we also think about it (helping) at a really, sort of, combative test match (situation) or under pressure... situations," Castle added. (Reporting by Ian Ransom; Editing by John O'Brien)
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