McLaren's two drivers on Thursday confirmed that the diffuser fitted to the rear of their MP4-25 cars has been altered since the 2010 season opener in Bahrain two weeks ago.
It is believed that Mercedes, Renault and Force India have also had to make the entry hole for the external starter smaller, following a clarification issued by the FIA.
It is potentially bad news for those teams, given that Ferrari and Red Bull were already leading the pace in Bahrain, and with the long distance to Europe meaning that improvements to the cars have been few.
"They (the engineers) said it doesn't make any difference," Lewis Hamilton said when asked in Melbourne about the altered diffuser.
Jenson Button, meanwhile, insisted that McLaren was not alone in having to make the change, and estimated the loss of performance at just "one or two hundredths".
"I don't really know," added the Briton, "but it isn't something that should cause us concern."
Meanwhile, a look inside the Sauber garage on Thursday revealed that its McLaren-style air inlet is located on the left-hand sidepod, as opposed to the monocoque-mounted 'F-duct' on the McLaren.
Red Bull's Sebastian Vettel confirmed that his RB6 is not fitted with one this weekend.
"It's not so easy to copy, it's pretty clever," said the German, albeit admitting that all serious teams will be working on a version.