An Air New Zealand flight to Australia has returned to Auckland Airport after an apparent technical issue.

Flight NZ101 took off from Auckland at 7.20am this morning and flew a short distance towards its destination at Sydney before being directed to return to New Zealand, according to flight tracking site Flightaware.com.

"The pilots operating NZ101 from Auckland to Sydney this morning elected to return to Auckland as a precautionary measure due to an issue with the system that heats one of the windows in the flight deck," an Air NZ spokeswoman said.

There was no risk to passengers, she said.

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"The aircraft landed without incident and customers are being accommodated on alternative services," she said.

It comes after another Air NZ service - flight NZ80 - last night returned to Hong Kong a short time after taking off.

The pilots elected to return to Hong Kong "as a precautionary measure due to fault with one of the hydraulic systems onboard", the spokeswoman said.

The aircraft landed without incident.

She said the issues aboard both flights were "separate and unrelated".

A source told the Herald the pilots aboard the NZ80 flight noticed a bang shortly after take-off from Hong Kong and found the aircraft was losing fluid in its central hydraulic system.

The pilots then had to dump 50 tonnes of fuel in order to land before the plane was towed off the runway.

The aircraft had been scheduled to fly out again from Hong Kong at 2pm today, the source said.

Air NZ had not yet replied to a request for comment, confirming this.