Along with its spurs, State Highway 20A and 20B, the state highway serves Auckland Airport, the country's largest, therefore making SH 20 a key arterial route connecting the airport to the wider Auckland region and most of the upper North Island.

It then runs over the Māngere Bridge to Onehunga, whereupon it continues northwest, cutting through Mount Roskill and Owairaka.

[4] In 1968, the Auckland Regional Authority began planning agreements for the route between the Northwestern Motorway and Dominion Rd, while the National Roads Board (now a part of the NZ Transport Agency) began organising the route southeast, between Dominion Road and Wiri.

[6] In May 1978 the construction halted when workers organised a labour strike over insufficient redundancy payments.

The partially constructed bridge was picketed for a period of two and a half years, becoming the longest continuous labour strike in the history of New Zealand.

[7] Eventually, the bridge was opened in February 1983,[6] and by 1984 the motorway extended south to Coronation Road.

[15] Work began on the Mt Roskill Extension in August 2005, with construction of the section ending at Maioro Street/Sandringham Road finishing on 15 May 2009.