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A reward may be offered to help catch the person who sabotaged erosion protection and weakened Dunedin's defences against a threatening Pacific Ocean.

After Dunedin City Council contractors worked through the night to get emergency sand to a rapidly eroding beach, a "very angry" Mayor Peter Chin said yesterday he would consider posting a reward.

He urged Dunedin people to contact police if they knew the culprit.

"This is serious stuff that threatens the city," he said.

"Sand sausages" - bags filled with sand and laid to protect sand hills from erosion - were slashed at St Clair beach this week, leaving the council in a race against time to protect the beach before the arrival of predicted southerly swells later this week.

The council invoked the emergency powers of the Resource Management Act to perform emergency works without resource consent - a move forced when the sea sausages were vandalised.

Senior Sergeant Phil McDouall, of South Dunedin, said investigation of the incident was continuing, but nobody had been arrested.

Dunedin city councillor Richard Walls yesterday called for the council to offer a reward of between $2000 and $5000 to catch whoever vandalised the sand sausages.

Mr Chin said he would consider using money from the mayor's discretionary fund for the reward.

He commended workers who turned up at short notice to halt the erosion and who were helping "because of the way they feel about the city".

Mr Chin was unsure what the cost of the work would be, or how it would be included in the council's budget just four days into the new financial year.

It was possible money in hold-over budgets for other work could be used for the beach repairs.

St Clair was visited by a sand-laden truck every five minutes yesterday as efforts to shore-up the bank between St Clair and Middle beaches continued.

Each truck carried 7cu m of sand and about 1120cu m had been shifted by early yesterday, about a 10th of what the council estimated it would need to get the job done.

The work, employing three diggers, was expected to take 10 days.

Some of the sand being dumped on the beach was being dredged from Otago Harbour.

Port Otago technical services manager Lincoln Coe said the suction dredge New Era would deliver at least two loads of sand a day. The wet sand needed to drain on the city wharf for 12 hours before being trucked.

Each load amounted to between 30 and 60 truckloads, depending on how much sand could be scooped out of the dredge's 600cu m hopper.