Firefighters have been called to an early morning barn fire in Christchurch for the second time this week.

The blaze broke out in a small barn on Sawyers Arm Rd in the suburb of Harewood about 2.30am.

Three pumps and a tanker have been at the scene with a number of crews.

It is unclear if the fire is related to a large hay barn fire near Brooklands, which broke out at a similar time on Monday morning.

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Fire investigators have been treating that blaze as potentially suspicious.

A spokesman for Fire and Emergency New Zealand, Andrew Norris, said there was no logical explanation for how Monday's fire started.

At that blaze, 21 firefighters had to pull apart about 150 large bales to dampen down hot spots.