Meanwhile after just seven days of an audit of every address in Thames Water's region - running in parallel with the amnesty - over 1,200 possibly unbilled customers have already been identified.
Thames Water said their details are now being checked and anyone that turns out to be using water without paying bills will face up to six years of back-charges, unless they come forward during the amnesty.
The audit and the amnesty were both launched on 20 January as the fairest way of ensuring all Thames Water customers receive the correct bills, and the company receives the income it needs to maintain and improve the service it provides. Under the amnesty if unbilled residential customers get in touch voluntarily they will not be charged for their previous water use. However, if they are discovered after the amnesty period closes on 3 March, they face back-charges for unpaid bills.
Mike Tempest, Customer Services Director, said:
"Of the 750 calls to our amnesty line in week one of our six-week amnesty, 357 have been from possibly unbilled customers who are set to avoid costly back-charges and start paying from now on for their water.
"However, after just one week of our customer base audit, 1,237 addresses are being carefully checked. If we find the occupiers are using water without paying for it, and they don't take advantage of our amnesty, the consequences could be costly, with back-charges due for up to six years.
"We're running this amnesty because, firstly, we want our bills to be as affordable as possible - and the extra income we get from currently unbilled customers will help achieve that.
"And secondly, we need an accurate picture of where our water goes in each part of our network: if it's going to customers, we want it to be recorded as such, rather than being unaccounted for."