The giant tunnel boring machine Alice has reached halfway on the first leg of her long journey underground at Auckland's Waterview Connection project.

She has constructed exactly 1.2km of the first of the 2.4km-long tunnels to connect the city's Southwestern and Northwestern motorways, the New Zealand Transport Agency said.

"Can we come and skate?" tweeted businessman Rod Drury when the NZ Transport Agency this afternoon released a photograph showing the interior of the vast new Waterview Connection tunnel.

"Or ride our jump bikes," tweeted another follower, likening the 10m diameter interior to the set of a sci-fi movie film.

Acting highways manager Steve Mutton said Alice had been almost unstoppable since resuming tunnelling after a planned two week-long maintenance shutdown in May.

"She has reached a snail's gallop, with 28 metres progress one day last week - her best day yet.

"That's not bad when your top speed is only around 80mm per minute."

Since she began her underground travels from Owairaka to Waterview last November, Alice had averaged about 4m per day excavating spoil and installing the tunnel's concrete lining.

"At those rates, the machine is well on schedule to complete the first tunnel in late September."

On her arrival at Waterview, Alice will be turned around over the following three months and re-launched on her southbound journey early next year, building the second (northbound) tunnel.

Construction of the two tunnels would be completed at the end of 2015, when they would be fitted out with the services needed to operate them, Mr Mutton said. These included ventilation fans, communication systems and fire protection. Sixteen cross passages - one every 150m - connecting the twin tunnels would also be built.

The entire project, which included the motorway connections either end of the tunnel, was due to be opened in early 2017.

By the numbers:

• Alice is the world's tenth largest tunnel boring machine;

• she is now 32.5m below the surface;

• 600 tunnel lining rings, each weighing 100 tonnes and comprising 10 pre-cast reinforced concrete segments have been installed;

• Alice has removed 197,000 cubic metres of spoil; and

• she has consumed $90,000 in power charges and $55,000 for water.

Galloping Alice reaches important milestone at Waterview http://t.co/gUEoo60PYE ^EB pic.twitter.com/Ab2arB8oPg

— NZTA Akld & Nthlnd (@NZTAAkl) June 13, 2014

- APNZ