The car left his property in Campbell St after 7pm on Wednesday.
Mr Kirby said the sighting was important as they believed the car was driven by Mr Round's killer or killers.
Police were also searching for items of discarded clothing linked to the scene, including a red long-sleeved jersey or sweatshirt, a black leather sleeveless vest, dark coloured stonewashed jeans, dark fingerless gloves and a pair of dark coloured sports shoes with a light colour around the soles and coloured laces.
Mr Kirby appealed to the public for more information about the movements of Mr Round before the attack and to look out for the clothes in the area that could provide vital information.
During his career spanning more than five decades Mr Round held roles as an editor and was foreign correspondent. In 2010 he was made a member of the New Zealand Order of Merit for his services to journalism.
He also once worked as a spy and had links to the KGB.
Family in shock over 'horrific' fatal attack
The former wife of journalist Derek Round says she is in total shock at his death.
The 77-year-old former foreign correspondent died in what police labelled a "horrific attack" at his Wanganui home. His body was discovered on Thursday morning.
Jan Round, who lives in the Wairarapa, had three children and four grandchildren with her ex-husband.
She said the news was "unreal".
"Total shock. You don't think this sort of thing is ever going to happen. He didn't deserve to die like that."
More details of Round's extraordinary life emerged yesterday, as his cousin, Martin Round, recounted how he became a double agent for the SIS in the early 1960s, informing on the Soviets.
Round was approached by Erick Lutskij, a diplomat with the Soviet embassy in Wellington.
"Lutskij asked Derek to provide him with information on various New Zealand matters about which he assumed Derek was familiar working as a parliamentary journalist," Martin Round said.
Lutskij subsequently passed Derek Round to Vladislav Andreev, who was the KGB resident in Wellington, and Mr Round met him and other Soviet officials at pre-arranged locations.
Round notified the SIS since he believed that in the Cold War atmosphere he had an obligation to assist the SIS.
The then director of the SIS, Brigadier Bill Gilbert, asked Round to play along.
Round was paid £30 a fortnight by the Soviets, which he handed over to the SIS. He was then given £20-25 a fortnight back. Andreev and another Soviet official were later expelled from New Zealand.
Mrs Round described her former husband as a gentleman, and said they were still very good friends.
She, her daughter Sally Laven, and son Mark Round travelled to Wanganui on Friday morning.
Search teams yesterday checked parks and reserves around Aramoho and also scoured the Whanganui River bank in an effort to locate clothing believed to be linked to the crime scene.
Another daughter, Susan Rebergen, wrote to the Wairarapa Times-Age : "My father's death is ghastly and unbelievable. I have been reading the news on all the New Zealand websites from [the Netherlands].
"It is hard to see all this happening from such a distance."
A funeral date has not been set but it will likely be held in Wellington.
