Police will no longer have trouble identifying a bank robber - he'll be the one in blue.

BNZ is installing spray units at its doors to douse fleeing robbers in a "synthetic DNA solution" that glows blue under ultraviolet light.

SelectaDNA spray, designed in Britain, stays on clothes for up to six months and on skin for one to two weeks and is invisible under natural light.

Each batch of the spray has a unique "DNA strand" that can link crimes to criminals.

It is similar to a system introduced last week in which property is marked with a coded microdot that can be matched against a database of registered owners.

Police have been given UV torches to shine on robbery suspects. A pinhead-sized spot of solution is enough to implicate a person.

BNZ branches in Counties-Manukau and Quay Park, central Auckland, are using the solution, and the bank plans to introduce it nationwide within months.

Each branch of the bank will be given a "DNA strand" unique to it.

BNZ security strategy manager Owen Loeffellechner said there was a small chance customers could be sprayed if they were near the doorway during a robbery.

"But burglars wait until the store is quiet anyway. Customers are usually frozen in place until the offender is ready to leave."

Mr Loeffellechner would not say how the spray would be released, but said there were "processes to go through when withdrawing money, and if that system is bypassed, it automatically activates".

SelectaDNA director David Morrissey said the spray lingered in the doorway for at least 45 seconds. Someone could run under it at full speed and still be covered.

He hoped it would be mainly used as a deterrent to robbers, rather than as a detection mechanism for the police.

"If someone is caught with this spray on them, we consider it a failure of the system," Mr Morrissey said.

If a person was caught with a spray-coating, the evidence was irrefutable.

If the case did go to trial, there was a "100 per cent conviction rate" as there was no way anyone could argue they were not at the scene.

* A spray is let off when security is breached, dousing the robber with a "synthetic DNA" solution.
* The solution, unique to each bank branch, glows under UV light.
* Police use UV torches to identify the solution.