The company, which provides fleet management services, acquired the car book of AssetCo Solutions' Northern Ireland arm.
The affiliation will also see Northern Bank, another subsidiary of National Australia, offering fleet solutions to its business customers through sister company, Custom Fleet.
Tony Haslam, head of Custom Fleet's UK business, said: 'Custom Fleet is delighted to have acquired this car fleet as it represents a first-class base on which to build our business in Northern Ireland.
'Custom Fleet will now be able to provide all forms of Fleet Management services for Northern Bank customers.'
Colin Dumas, general manager for Business Financial Services at Northern Bank, added: 'This acquisition completes and complements the range of services that Northern has been able to offer its business customers, from hire purchase financing through to custom fleet management.'
The news follows AssetCo's recent acquisition of Fleet Solutions (Ireland), the commercial fleet management organisation serving Northern Ireland and the Republic of Ireland's public and private fleet operators (Fleet News November 30, 2003).