We’ll begin rolling out this change today across our Android and iOS apps, on twitter.com, TweetDeck, and Twitter for Mac. It will continue to roll out worldwide – including Australia – over the next few weeks. If you can’t wait to try out longer Direct Messages, be sure you’re using the latest versions of our apps so you get the update right away. Sending and receiving Direct Messages via SMS will still be limited.
Twitter is taking on the messaging apps, after down-playing DMs for years. Hopefully we’ll see this come to third party apps, along with group DMs, very soon.
Source: Removing the 140-character limit from Direct Messages | Twitter Blogs