Auckland's popular Northwestern Cycleway running for 12km between Newton Rd and Te Atatu is to gain a 1.2km upgrade through its Kingsland sector.

Transport Agency contractors will start work tomorrow building an off-road section for about $3 million beside the Northwestern Motorway between Bond St and Myrtle St.

Cyclists have until now had to negotiate hilly backstreets to get between off-road stretches of the cycleway, which is the most popular in the Auckland region with an average of 513 trips a day.

The new section, which the agency expects to complete in two to three months, will allow cyclists to keep away from motor traffic for most of the 12km route apart from at motorway interchanges and around Carrington Rd.

Transport Agency regional highways manager Tommy Parker said investigations were also starting in co-operation with the Auckland City Council on how to extend the cycleway eastward, from Newton Rd to Symonds St.

But that would involve getting cyclists past Spaghetti Junction, so could take about two years to complete.

The cycleway will be upgraded between Waterview and Rosebank Peninsula from about 2011 when the agency widens and raises above floodwaters a marine causeway which it shares with the motorway.

That will be part of a project to widen the full length of the motorway west of St Lukes for about $860 million over more than 10 years, when the agency says the cycleway will ultimately be extended all the way to Westgate, to join an Upper Harbour route to Albany.

Mr Parker said the agency would pay the full cost of the Kingsland cycleway upgrade, as it would be part of the motorway corridor, but it would be managed by Auckland City.