Mercedes' engineering director Aldo Costa says the team has shown 'very little' of its W06's car potential so far.
The Brackley team dominated the first season of the new turbo V6 rules in 2014 and is expected to maintain that advantage this year.
But as testing kicked off at Jerez last week, it was Ferrari that set the headline times amid rumours it has found between 40 and 80 horse power over the winter.
Amid suggestions that Mercedes was actively sandbagging at Jerez, Costa told Gazzetta dello Sport that Mercedes displayed "very little" of the car's potential.
"But I think it is the same for us as it is with all the other teams," he added.
"Only at the first race in Melbourne will we understand the true situation. And maybe not even there; we may need to wait four or five grands prix."
Differences are on the inside
Although the new W06 appears at least visibly very similar to the 2014 car, Costa said there are many new things about it.
"The car is an evolution," Costa said. "But in reality it is very different in many parts that cannot be seen from the outside."
As for Jerez, he said the test was "positive in all respects" for the team, even if more work needs to be done on reliability.
"The improvement compared to 2014 was in line with what we expected," added Costa.
"We did not go to Spain to verify performance, but rather (to verify) reliability and the behaviour of the car."
Costa refused to be drawn on how many unfreeze 'tokens' Mercedes has already spent on its 2015 engine, but he did reveal: "I can confirm that we are using our new power unit for 2015, which is different from the last one."