Melbourne Rebels appeared on course for victory at the interval thanks to a Ged Robinson try and the boot of Wallabies star James O'Connor.
But the Waratahs stormed back with touchdowns from Bernard Foley, Michael Hooper and Paddy Ryan.
Beale snatched a try at the end to secure a bonus-point but it was merely a consolation.
The Waratahs struggled with the conditions in the first half and they were dealt a blow when Tatafu Polota-Nau was yellow-carded for a ruck offence.
And Robinson raced into the corner after a smart lineout move that saw the ball returned to the hooker
"We probably made it hard for ourselves but we kept playing," said Waratahs captain Dave Dennis.
"It was poor discipline really. We had to keep backing ourselves and not look at the scoreboard."
But Foley crossed following a break from Kane Douglas, while Hooper raced over following a slick move.
Ryan put the result beyond doubt after crashing through a tackle for his first try in Super Rugby.
An injection of pace from Beale exposed some tired Waratahs defence but the game was already wrapped up.