Bottas benchmark fastest pre-season lap was struck down from the timesheet this morning when Sebastian Vettel went quicker on the penultimate day.
"We are definitely not confident that we are in front," said Bottas.
"We are not relaxing, we are not in a comfortable situation at all. We do feel that the other teams have made good progress over the winter.
"Ferrari are looking very strong, Red Bull can always surprise, and other teams can be strong. But Ferrari look solid, strong and fast - and who knows what people will bring to Melbourne.
"We should not underestimate them at all. We're not saying we're number one, we're just working flat out."
Bottas was impressed by Vettel's morning effort on the Pirelli's UltraSoft compound, especially as the German eased off on the straight, thus sandbagging his lap.
"A very fast time," Bottas admitted.
"It's difficult to know what type of things they were running and how much they were taking out of their car but a respectable time and very quick. Definitely, we shouldn't underestimate them."