National Australia Bank has announced the purchase of Hong Kong investment advisory firm Calibre Asset Management for less than A$5 million ($4.6 million).

The acquisition, although small, complements NAB's expanding private banking network. Calibre Asset Management is a well-respected boutique advisory firm that provides investment strategies for a select group of clients in Hong Kong, as well as  discretionary portfolio management services.

"There are synergies between Calibre and the personalised banking services we've offered in Hong Kong for more than 30 years," said Andrew Macintosh, the general manager of banking for NAB Hong Kong, in a statement.

"Calibre's ability to provide clients with advice and choices in managing their money will build the breadth and quality of the offering for our private clients," he added.

Calibre will form a subsidiary of NAB following the acquisition. However, it will still retain is customer focus and remain a Hong Kong firm.

Earlier this year, NAB Private Wealth, the private banking business within MLC & NAB Wealth, today announced the creation of a new wealth services division.