"I wouldn't sit here and bet my house on us winning the World Cup," Fisher said on Tuesday after confirming he'd finish up as forwards coach next month.
"That'll be the aim and that's a huge ask. If you say where we are now, it's certainly a massive hill to climb."
Ranked eighth in the world and coming off a first winless four-Test European tour since 1958, the Wallabies face a daunting World Cup draw.
The tournament hosts will play arch rivals New Zealand in the group stages of the global showpiece for the first time and, if they conquer the All Blacks, will likely run into two-time defending champions South Africa in the quarter-finals.
"Is there quality? Yes, there is. Are there good coaches? Yes there are. Is there depth? It's growing," Fisher said of Australia's prospects.
"But you look at New Zealand, you look at South Africa, you look at France, you look at England, you look at Ireland and all those other teams, like the top 10, 12 teams - it's tough.
"So I wouldn't sit here and bet my house on us winning the World Cup, but I would bet my house on everybody involved giving an absolute red-hot go and having belief that they can."
While the refreshing Fisher was being typically realistic with his assessment of Australia's hopes of lifting the Webb Ellis Cup for the first time since 1999, the veteran mentor isn't completely giving up the ghost.
But after seeing the Wallabies leak an average of 34 points a game on last November's spring tour, Fisher says Australia's incoming defensive coach Scott McLeod has a huge task ahead.
"I don't know what the new defence coach is going to change. I haven't had discussions with him, but a tweak's a good thing. We haven't set the world on fire," Fisher said.
"There's obviously changes and improvements that could be made and hopefully the guys that are coming in are scrutinising that and have ideas - and I'm sure they do."
Fisher believes that while the Wallabies must improve in all areas to challenge for World Cup glory, the side's performance would likely come down to individuals stepping up.
"It's about being absolutely familiar with your game and committing to your game," he said.
"We spoke this morning, it only takes one person to let you down in any particular thing. You can have the best defence system in the world. If one person knocks off, that's what creates opportunity. It's not system.
"We need to make sure that we drive and the players buy in 100 per cent. If you make a mistake, you make a mistake. But you never make an effort mistake.
"I do believe looking at Super Rugby that even Wallabies just have too great a capacity to blow hot and cold."
A long-time former Brumbies coach, Fisher is intent on remaining in the game, hoping to work as a consultant for Rugby Australia or one of the nation's four Super Rugby sides.
More immediately, he opted to step aside to care for his 93-year-old father.
Fisher will finish up after Australia's three Nations Championship Tests in July, with unrelated Englishman Jonny Fisher joining Les Kiss as an assistant.
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