The plates on the getaway car used in the smash-and-grab theft of two Gottfried Lindauer paintings appears to be fake.
Inquiry head, Detective Inspector Scott Beard told the Weekend Herald the number plate on the late-model White Holden Commodore SSV did not match any on the New Zealand vehicle registry.
"There is a possibility it could have been altered. We are looking at those sort of possibilities.
"We are looking at all [similar Holdens] that have been reported stolen and at that type of car in general," Beard said.
Police are seeking three men caught on CCTV leaving in the Holden soon after a stolen utility vehicle reversed through the front window of the International Art Centre on Parnell Road, Auckland, about 4am on April 1.
Footage shows two men stealing the paintings, which were displayed in the gallery window, and escaping in a Holden driven by a third man.
Beard said detectives were working through information from the public that followed the release of CCTV images of two of the three men.
The two portraits, of Chieftainess Ngatai-Raure and Chief Ngatai-Raure, date from 1884. They were about to be auctioned, and together were estimated to be worth $1 million.
Art conservator Sarah Hillary appealed to whoever has the paintings to take care of them.
Any damage would be very difficult to repair because Lindauer painted thinly and the surfaces of his works are very regular, she told the Herald.
"You can see the canvas texture overall and any disruption to it is very obvious.
"If there was dirt on the surface, we might be able to take that off. But when something is torn, even though we can repair it, it is always going to have that damage."
Paint would have been lost and the structure weakened, she said.
Hillary spent two years restoring the $2 million James Tissot painting, Over The Top, stolen in 1998 by a gun-wielding thief.
Damage invariably reduced the value, she said, because the value was not only in the beauty of the work but also because it was an historical object.
"I would appeal to whoever has them to look after them because the more damage the worse it will be. They are old paintings and they are very fragile."
Czech-born Lindauer trained at the Academy of Fine Arts in Vienna and migrated to New Zealand in 1874.
He became one of the most prolific and best-known painters of Maori subjects along with Charles Frederick Goldie.
Publicity and interest in the theft would make the painting impossible to sell, art recovery expert Christopher Marinello previously told the Herald.
"These paintings are now radioactive - no one in their right mind will touch them."
Marinello, who is based at Art Recovery International's office in Venice, has helped recover more than $500m of stolen art.
The nature of these thefts suggested it was an unsophisticated job and carried out by people who thought they would be able to sell the paintings quickly, he said.
"This was not an elegant robbery ... We are not talking about stolen to order because of the way the smash and grab was done."
Most works were recovered because it was harder to sell stolen art than it was to take it in the first place, he said.
"There is a bit of a black market of course but they are offered for only about 5 per cent of their value."
But Marinello said there was a market for the Lindauers overseas because they were attractive works of art. He urged New Zealand authorities to register the theft and details of the artworks on the Artive.org register, which he said was the most technologically advanced system in the identification of stolen art.
It is unclear whose ancestors Chieftainess Ngatai-Raure and Chief Ngatai-Raure are. International Art Centre director Frances Davies said all they knew about the paintings was in their catalogue online.
Detectives are considering all motives, said Beard. Border authorities were alerted soon after the theft occurred and the art community was informed via Interpol.
Both men caught on video are estimated to be in their 20s or 30s.
One is described as fair-skinned, of medium build. He wore a black sweatshirt with white logo on the front, a black cap, red or orange bandanna, black shorts and gloves, and black shoes with a white sole and red or orange lacing.
The other, of skinny to medium build, wore a black hooded zipped sweatshirt, black track pants with a white stripe down both sides, dark coloured shoes with white soles and a light coloured bandanna.
Beard requested that anyone who recognised the men or had information about the crime or where the paintings were contact Auckland City Police on 09 302 6832 or Crimestoppers on 0800 555 111.
- Additional reporting by Kirsty Wynn