Renault are on course to make substantial gains with their 2015 Formula One power unit before the season-opening race in Australia, according to chief technical officer Rob White.

The French manufacturer made sweeping changes to their power unit over the winter in a push to reduce a 2014 performance deficit to Mercedes.

And while reliability issues hampered Red Bull and Toro Rosso's running in this year's first pre-season test at Jerez, Renault are confident that big strides will be made before the first race in Melbourne on March 15.

"First and foremost we need to run reliably, be quick and closer to front," White told Renault's F1 website.

"Our honest expectation is that we will make a decent improvement but it is difficult to quantify the gain relative to our competitors who will also progress.

"What we can say is that we are on course to make a significant performance step and resolve the principal reliability weaknesses by the time we get to the first race."

White added that Renault had opted to use most of their engine development tokens prior to the 2015 Formula 1 Rolex Australian Grand Prix in order to deliver big performance gains - but said deciding future upgrades is a "juggling act".

"We have used the majority of the tokens for the first race and our use of tokens during the course of the season will be relatively modest," White explained.

"It then becomes a matter of strategy about when you introduce the remaining tokens; whether to introduce at the start of the season when the technology is relatively immature but could give greater relative performance, or later in the season when the part has had more testing miles but the impact on performance will be potentially less.

"Clearly the juggling act we need to perform is which areas of the power unit are the most worthwhile to attack for performance reasons. We have therefore prioritised token spend to make as much headway as possible with performance."