Key Points:

Motorists will be offered seamless motorway-to-motorway connections from Auckland Harbour Bridge to the port and to Waitakere City from Wednesday, as Spaghetti Junction nears the end of a $200-million-plus refit.

New links the other way - from Waitakere and the port to the bridge - will open a fortnight later on December 18 and 19, easing congestion from traffic which has until now had to grind through central city streets to reach the Northern Motorway.

"Hurray, no more Quay St," cheered a port official on Friday, in anticipation of an easier passage for northbound trucks from the waterfront to the motorway via Grafton Gully, replacing their traditional struggle through downtown Auckland to the Fanshawe St on-ramp.

Neither will drivers have to dog-leg through Union St to get from the Northern to Northwestern Motorway, or vice versa.

At least, that was the theory behind adding the four new links to what Transit NZ prefers to call the Central Motorway Junction, a fiendishly complicated, multi-layered puzzle of concrete, steel and asphalt carrying 200,000 vehicles a day.

In practice, Transit suspects some drivers may keep using Union St and the Wellington St on-ramp to get ahead of any queues of northbound traffic heading to SH1 along the new direct link from the Northwestern Motorway.

"They may choose to keep going down Union St to get the jump," said project manager Colin Holtshousen.

But both the old Wellington St entrance and the new motorway-to-motorway connection a little further south will be regulated during afternoon peaks by lights called ramp signals, which will release vehicles at measured intervals into the mainstream State Highway One traffic.

The junction's new links to the port and Northwestern Motorway will not have signals at the outset, as they are expected to be relatively free-flowing, but Transit plans to spend $50 million extending the technology to 61 Auckland motorway on-ramps over two years.

Vehicles bound for the port or western suburbs will be able to leave SH1 from the left, just before the Wellington St bridge. They will travel along a new ramp which will cross above the motorway before running alongside it, to the right, then splitting into two carriageways and joining traffic from other directions.

The westbound carriageway will join traffic from central Auckland's Hobson St on-ramp to the Northwestern Motorway, and the other will merge with the old Northwestern-to-port motorway ramp before running down Grafton Gully.

Conversely, vehicles heading north from the port will split from west-bound traffic up a ramp before merging with a new carriageway heading from the Northwestern Motorway and running under the Karangahape Rd bridge.

Mr Holtshousen says the four-year project, undertaken in two stages, has been a gruelling struggle of "overs and unders" in which engineers have managed to snake and squeeze the new links through a devilishly tight corridor while ensuring drivers will have adequate and safe sight-lines.

Overall project costs have amounted to $207.5 million, including a $55 million first stage which involved adding a new lane along the Southern Motorway between Hobson St and Gillies Ave, and creating more space on the 204m Khyber Pass Viaduct by turning it from a double structure into a much stronger single one.

The second stage included building new off-ramps to Nelson St from both the Southern and Northwestern motorways, allowing traffic to exit from the left instead of the right.

But the cost has not included a new fifth lane added to the Northwestern Motorway between Newton Rd and Western Springs for $9 million, nor the Grafton Gully motorway branch which was completed three years ago for $67 million.

State Highway 1 has been closed through the junction more than 400 times at night over three years, and the links between the port and the Northwestern Motorway have been closed almost 600 times, in a huge logistical exercise aimed at keeping the corridor open for day-time traffic.

The Figures

* $207.5m Total project costs

* 750m New viaducts and bridges

* 143,000 Plants on banks beside concrete structures decorated by depictions of karaka leafs and berries and pohutukawa flowers.