Three bushwalkers have spent a freezing night lost in a state park north of Melbourne with only a child's hoodie and makeshift structure to shield them from the cold.

Kirrilee Ord, 21, her brother Mitchell, 12, and boyfriend Ben Warlond, 24, from Werribee were hiking in Lerderderg State Park, near Bacchus Marsh, on Sunday when they became lost.

Found after a night in the bush: Mitchell Ord, Kirrilee Ord and Ben Warlond with dog Sasha. Credit:Justin McManus

The trio were all wearing shorts and T-shirts and had taken only one packet of Twisties to share on what was planned to be a two-hour walk.

Kirri's half-sister Chelsea Newby raised the alarm just after 4pm when they failed to return to their campsite.