Thousands of people have lined the streets of central Napier for this afternoon's Art Deco Parade.

But many did not notice Prime Minister Jacinda Ardern, an 11th-hour inclusion in the hour-long, sun-swathed parade after her acceptance of an invitation was announced only late on Friday afternoon.

She was dressed in sunfrock and hat, but visible mainly to those on the southern side of the difficult path through pedestrian-vehicle CBD thoroughfare Emerson St as she rode in the open-aired rear seat of Wanaka vintage luminary Robert Duncan's 1918 Packard V12.

Beside her were Art Deco-attired Isabella and Will, children of Napier MP Stuart Nash, closer to the hills-side of the street where the major clue to her close proximity was the wired-up protection squad member seated beside the driver.

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Despite the almost-unbelievable road and foot traffic into, in and around Clive and Memorial squares, Emerson St and Marine Pde, the parade was able to start on time with the PM and the Packard about 10-back, when many might have expected her to be at the head of the parade, lead by perennial front piece, the 1937 Austin 7 of Art Deco ambassador Bertie attended immediately by the Navy band.

The V12 was one of over 200 vintage vehicles and bikes in the parade, which attracted the biggest crowd in the 31 years of the Art Deco Weekend and Art Deco Festival celebrating the era of Napier's emergence from the horrors of the 1931 Hawke's Bay Earthquake.

At least 25,000 lined the footpaths of Emerson St, the fold-up chairs of the early birds effectively picket-fencing the kerbside, and 5000-10,000 more thronged side streets and the Soundshell on Marine Parade where the parade ended, where the gleamy and shiny parked up, and from where Ardern went to the Mayoral luncheon soon afterwards.

The rest of the afternoon was expected to be PM own-time, but Ardern and partner Clarke Gayford were overnighting in Napier, enabling Gayford to judge the Deco Dog Parade at the Soundshell from 9-11am on Sunday.