Formula One champion Lewis Hamilton insists Mercedes have "work to do" as they prepare to start their title defence at the Australian Grand Prix on Sunday.

Mercedes have claimed the past five constructors' championships amid Hamilton's dominance of four world titles, sandwiched in between former team-mate Nico Rosberg's 2016 triumph.

Ferrari have not topped the standings since 2008, with Kimi Raikkonen the last driver from the Italian manufacturer to win a championship a year earlier.

However, Sebastian Vettel's Ferrari are seen as the early team to beat ahead of this weekend's season-opener in Melbourne and Hamilton knows Mercedes have their work cut out.

"I don't think it's difficult to read it, I think it's quite clear," the Briton said on Thursday. 

"However, it's difficult to know what everyone is doing, naturally we won't fully know until we get out in the car tomorrow and come qualifying you start to get a better picture. 

"Usually over the first few races is when you really get an idea of where everyone stands. We said that we have work to do, we weren't talking BS [bull****], we have work to do.

"Naturally after a test you always improve on the next days, the car gets older and older throughout the test.

"It wears and then you go away and analyse where you've been, new information gets put in the wind tunnel, engineers have new information and they work on it. 

"Everyone takes a step after testing. The car will be fresher here, everyone will have a much better understanding of it. I think it will be the same for everyone. 

"It'll be interesting to see how Red Bull's upgrade worked and I know Ferrari usually bring an upgrade to the first race so it will be interesting to see."