A tunnelling machine named after an Olympic gold medalist is powering its way through downtown Auckland as part of the City Rail Link construction project.

Valerie, named after shot putter Valerie Adams, has completed the first 290 metres of tunnels between Victoria and Sanson Street.

It's being refurbished before it finishes the final 200 metre stretch towards Wellesley Street, which is due for completion in April.

A smaller tunnel boring machine is also working under Victoria Street, where crews are diverting and strengthening the Orakei Sewer Line.

In a statement, the City Rail Link project team said the stormwater work can begin before the tunnels are due along Albert Street.

"Construction on CRL which will link Britomart and the city centre to the western line near Mt Eden began in December 2015," it says.

"Every milestone takes Auckland closer to a new future for transport."

The twin rail tunnels will travel uphill from Britomart Station at the waterfront to Mt Eden Station in Eden Terrace.