A first half score from Garyn Smith and a penalty try, both converted by fly half Luke Price, gave Wales a 14-0 lead at the interval with Josh Adams adding a third try minutes into the second half.
At 19-0 up, Wales looked dominant but when flanker Jordan Viggars was yellow carded, Australia came back into the game. Back-rowers Jack McCalman and Lolo Fakaosilea both crossed for tries and with a conversion added, there were now just seven points between the teams.
Wales had the last laugh though with the impressive Tyler Morgan adding a fourth try to secure the win.
"I'm extremely pleased. In the first half we played some lovely rugby but I've got to admire the character of the team, the spirit they showed and their defensive qualities," said Lewis.
"It was excellent. I think it's part of their development to be comfortable with ball in hand and playing at a high tempo. I thought we showed a bit of naivety at times, we tried to keep the ball alive, possibly going against my philosophy, but I thought the way we approached the game defensively was magnificent.
"When we play against sides like Australia, it's such an education for the boys. To play South Africa in the summer and now Australia, it's been a great experience.
"In the past year a lot of these boys have played something like nine internationals. Several are likely to move into the Under 20's but a number of them will come back with us after Christmas."
Captain Jon Fox was a key member of a Wales pack which fronted up to some giant Wallaby forwards.
He added: "I thought we defended really well, not conceding any points in the first half especially. In the second half, our defence slipped a little bit and we let them back into it with a couple of tries.
"But our discipline was much better towards the end and it was a really physical game. Allan's motto for us is challenge teams with ball in hand and I think the backs did just that.
"They were all outstanding but I thought the pack were superb in providing a platform for them."
Wales: Tries: Smith, Penalty, Adams, Morgan; Convs: Price (3)
Australia: Tries: McCalman, Fakaosilea; Convs: Deegan
For a full match report from last night's match click here.
For full match highlights click here.