Leo DiCaprio -- Gas Guzzler Down Under

Leonardo DiCaprio may have reason to hide ... because the environmentalist wasn't very green as he arrived in Sydney Monday ... on a private jet.

In the past, Leo's camp has claimed the 37-year-old "J.Edgar" star occasionally flies private when he has to get to remote locations.

Sydney may have kangaroos, but it's not remotely remote.

A source close to Leo tells TMZ ... he simply hitched a ride with a plane that was already headed to Australia.