Kellan and Sharni's Sunny Holiday

These two are heating things up-- both literally and figuratively!

Twilight hunk Kellan Lutz and his girlfriend, Australian actress Sharni Vinson, stripped down to their skivvies while on vacation in Vinson's hometown of Sydney this week.  The loved up couple took advantage of the warm weather by spending most of their time outside-- whether it was walking the dog or goofing around on Cronulla Beach.

Things are getting serious for these two-- do we hear wedding bells soon?

EXCLUSIVE photos by SNAPPER