Mera appears to have a new battle suit for Justice League as revealed by a tie-in toy for the film.
Several other tie-in toys for Justice League have included Mera in the past, but this is the first time that she dons a different outfit from her iconic green battle suit.
As previously mentioned, Aquaman is still doing principal photography in Australia under the codename Ahab.
Given that Justice League is an ensemble flick and the character presumably a supporting one, it is difficult to justify having her wear two different sets of clothes if her appearance would only be limited.