Prime Minister Bill English said the Waterview tunnel project was 60 years in the making.
"You can make roads look ugly but you can make mayhem attractive to the eye," Mr English said at the Mt Roskill end of the tunnel.
Transport Minister Simon Bridges said today marked a pivotal milestone not just for Auckland but New Zealand's highway network.
Other signatories attending the opening include Auckland Mayor Phil Goff and Mt Albert MP Jacinda Ardern.
This will be followed by the public walking and cycling through the twin, 2.4km tunnels.
The tunnel completes the 48km western motorway ring route - a second motorway route through Auckland - and includes a giant motorway interchange at Great North Rd to connect the southwestern and northwestern motorways.
At five open days over the next 10 days, 60,500 people will have a once in a lifetime opportunity to walk and cycle inside the tunnel, with its 140,000sq m of paint - black for the roof, white for the walls - 4000 lights and 400km of cabling and wiring.
Both tunnels were bored by a giant tunnelling machine, named Alice. At 2.4km long, Waterview takes the record off the 1.97km Lyttelton tunnel as the longest road tunnel in New Zealand.
