Inquiries are continuing after a suspicious fire destroyed a shed in Wairoa early on Saturday.

The blaze was discovered on Mahia Ave at 3.36am by Wairoa firefighters returning from a small veranda fire in Ruakituri.

Wairoa Volunteer Fire Brigade chief fire officer Grant Duley said the shed was "pretty well gutted" when they arrived and several household items inside were damaged.

The adjacent house, which had no one living in it, had to be doused with water to protect it while they waited for another appliance to help extinguish the fire, he said.

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"It was lucky we were going past and saw it.

"We were the ones to put the call in, which is not common."

Firefighters spent 45 minutes at the address and called police because they believed the fire to be suspicious, Mr Duley said.

A police spokeswoman said police attended at 4.05am and were treating the fire as suspicious.

Inquiries are ongoing to determine the cause, she said.

The fire crew had been returning from a property on Ruakituri Rd following a small veranda fire which had been extinguished by the time they arrived.

Occupants of the house had put hot ashes from their indoor fire on the veranda the night before which ignited by 2.15am, Mr Duley said.

The residents used buckets of water to extinguish the flames with only a small part of the veranda receiving fire damage.