Builders are being told to take extra precautions to protect their clients after a scam was uncovered that diverts invoice payments to dodgy dealers.

Hackers have gained access to the email accounts of some builders and have altered bank account numbers on invoices destined for clients.

The client then pays the bill and only realises they've been scammed when the builder follows up on outstanding payments.

One woman lost $14,000 after falling victim to the scam.

Consumer New Zealand researcher Jessica Wilson said builders need to be careful.

"They should be looking at their internet security, making sure it's all up to scratch and they're protected," she said.

"They may even revert to sending invoices the old-fashioned way, by mail."

Wilson said those expecting a bill from a builder also needed to be savvy to avoid losing out.

"Be very vigilant. If you're expecting a big bill from your builder, ring them up, double check the account number."