Air New Zealand has traced a problem that forced a Dreamliner to return to Auckland Airport.

Emergency services were scrambled on Saturday after reports of a smell ''like electrical burning.''

Flight NZ103 was bound for Sydney before it had to return shortly after take-off.

A spokeswoman said the smell on the aircraft was caused by an issue with part of the air conditioning system.

The aircraft returned to service later the same day.

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That incident followed a problem on July 12 when another Sydney-bound Air New Zealand Dreamliner flight was aborted shortly before take off.

In that case a strong smell of burning was detected on board the aircraft and some passengers reported seeing fire and smoke coming from one of the plane's engines.

The plane, which was carrying 289 passengers, returned to the gate.

Air New Zealand chief flight operations and safety officer David Morgan said then that following extensive testing it was determined what was being observed was a normal vapour which is released by this model of engine.

In some atmospheric conditions it could be more pronounced.

''The safety of our customers, employees and aircraft is of utmost importance to Air New Zealand. We appreciate our customers bringing this to the attention of our crew so we could properly investigate the cause, '' he said.