They must turn off State Highway 16 at Brigham Creek Rd and drive past the Whenuapai Air Force Base to reach an interchange halfway along the new motorway.
That is because the Northwestern Motorway extension will have no off-ramps between Brigham Creek Rd and Lincoln Rd in Henderson until the Transport Agency is satisfied population growth can justify the expense.
The lack of access to northern Westgate is frustrating Auckland Deputy Mayor Penny Hulse, a Waitakere resident who welcomes the new motorways but says the agency is failing to make the most of its investment.
"We've had a long battle trying to get them to understand that this huge development, critical for the Auckland region, needs off-ramps so people can get to it," she said.
"Their argument was, we'll wait and see how it goes and then we'll put in off-ramps, but if you don't build the off-ramps how are people going to get to it?"
Labour MP Phil Twyford says the lack of motorway access is bizarre and undermines the viability of a development tipped otherwise to create 10,000 jobs and add $2 billion a year to the economy by 2051.
He has written to Prime Minister John Key, who is the local Helensville MP, asking for help.
"The intent ... is to generate jobs in the west so the region's people don't have to commute long distances across the city, putting huge strain on Auckland's transport infrastructure," he told Mr Key.
"Yet NZTA [the Transport Agency] sticks ... doggedly to the idea that the new motorway is fundamentally a bypass between State Highway 1 and the Auckland isthmus, and they don't seem to think it should serve this massive new commercial hub."
He accused the Transport Agency and Transpower, which is burying power pylons through the site at a cost to the property group and the council of about $18 million, of being obstructive and unco-operative.
Mr Key's office would not comment, but both organisations reject the claim.
Transport Agency highways manager Tommy Parker said his team was showing goodwill towards the development by building the bridge pier and investigating prospects for adding off-ramps.
"We haven't found a good geometric layout that allows that to work and we are not sure we can physically fit them in."
NEW MOTORWAYS
* Westgate to Hobsonville - 6km.
* Northwestern Motorway extension - 3km.
* Total cost - $220 million.