A Panmure car yard fire is under control but traffic is backing up as firefighters continue to clean up the site.

A witness said the fire had broken out at DNA Motors on the corner of Ireland Rd and Lagoon Drive.

The road had been closed off at the roundabout.

"From Pakuranga heading to town it's causing a backlog of traffic, heaps of rubberneckers stopping to see what's going on," he said.

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A Fire and Emergency spokesman said they were called to the fire at 7.03am and two fire engines attended.

"It was a fire in the roof of the building. Nobody was inside."

They asked police to help control traffic at the roundabout as firefighters tackled the blaze.

The cause of the blaze is unknown.

Traffic is backing up on most of Auckland's citybound motorways because of crashes and breakdowns this morning.

The NZ Transport Agency said traffic was heavy on the Northern Motorway citybound between Redvale and Onewa Rd, but free-flowing northbound.

On the Southern Motorway it is heavy between Drury and Takanini citybound, and in patches between Manukau and Mt Wellington because of a crash just before Princes St, which has been cleared.

It is moderate to heavy between Ellerslie and Greenlane, and moderate southbound at Greenlane.

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On the Northwestern Motorway it is heavy citybound between Westgate and Royal Rd and between Te Atatu Rd and Bond St.

An earlier crash at the Great North Rd interchange has been cleared.

There is also moderate-to-heavy congestion northbound on the Southwestern Motorway between Puhinui Rd and Bader Dr after a now-cleared breakdown.