The driver told reporters at a pre-season launch that he was under team orders to get his weight down and still had some way to go if he is to reach the target before the first race in Melbourne on March 20.
"I'm still getting there. So far I've lost just over three kilos and I've got about two more to go," said Ricciardo, winner of three races in his career.
Driver weight was a particular issue during the 2014 season when the significantly heavier new V6 turbo hybrid power units were introduced to replace the old V8 engines.
Bigger men like Ricciardo, who is 1.75 metres tall and tipped the scales in the mid-60kg range in 2014, had to lose weight then because minimum car weights did not increase accordingly.
The limits were increased by 10kg for the 2015 season to level the playing field and address concerns that drivers were risking their health with too much weight loss.
However, Red Bull were handicapped last season by their uncompetitive Renault engine and are now seeking other ways of closing the performance gap, one of them being reducing weight where possible.
Weight: A contentious issue for drivers
Teams believe that every five kg of weight is equal to around 0.2 seconds per lap, a significant amount in a sport measured to the thousandth of a second.
"It is not fun losing weight but if I thought it would be a real challenge...I would not have got the weight off," said Ricciardo, making light of the task.
"I know I can do it and hopefully get myself there. It is not only weight I put on over Christmas because I didn't put 5kg on," he added.
"But we are quite tight with the weight of the car and we have been advised to be a bit leaner than last year. So it will be more like 2014 for us now."
Ricciardo's Russian team mate Daniil Kvyat, ever competitive, joked that he had lost 25 kilos.