Monk's representative, Kacey Spies, said that Madden proposed over the holidays, People Magazine said Tuesday.
"Benji proposed to Sophie in Australia on Christmas Eve with a Neil Lane diamond ring," said Spies. "They are very excited and enjoying this time with family and friends."
The couple was first spotted together kissing in September in Los Angeles.
Madden's twin brother, Joel, told People of Monk, "I love (Benji's) girlfriend. She's so cool. I hang out with her, we have a great time. She's really similar to me -- she's very care-free."
"(Love is) the most rewarding thing in the world. It makes everything else in your life more rewarding. Your work, your family. Home, everything. It's great. It's a singer/songwriter thing -- we're just hopeless romantics," added Madden.
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