Officials will dig deep to find the cause of two burst water mains in two days, affecting an estimated 150 homes in Whangarei.

The latest pipe to burst was on Whareora Rd, and was reported to council at 7.20am yesterday.

Whangarei District Council water distribution engineer Simon Charles said an estimated 50 houses at the Mill Rd end of Whareora Rd were affected.

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It was initially hoped the water could be back on by 11am, but contractors were delayed after large rocks made it difficult to excavate the damaged pipe.

He said there was a longitudinal crack along the pipe, and 3m of pipe was replaced. Water was restored at 12.30pm.

This break followed another break in the water main on Vale Rd in Riverside at about 7.40am on Wednesday.

Mr Charles said the water was restored later the same day.

He said the broken pipe affected about 100 properties on Vale Rd and a few of the surrounding roads. A 6m section of pipe was replaced.

Mr Charles said the pipes would be tested to determine what had caused the breaks, but there were a number of factors to be considered.

Those included hydrant testing which can cause water hammer - the sudden opening or closing of the valve can send a shock wave through the pipe.

Breaks can also be caused by valves being opened and shut as part of maintenance, faulty variable speed drivers in the water pumps - which could cause sudden bursts of water - ground movement, and some soil conditions softening the pipes.

He said it was unlikely the recent wet weather had contributed to the recent pipe breaks.

Mr Charles said the two in two days was a coincidence, and the council usually averaged four main pipe breaks a month.