Water restrictions are a matter of when, not if, for Invercargill residents.

The Invercargill City Council's calling for locals to turn down their garden hoses to help preserve their water supplies.

Forecasters are predicting a long and hot summer in the south, and council water manager Alister Murray said restrictions aren't far away if the rain stays away.

He said the Oreti River levels have dipped below the minimum levels already.

"We have had a little bit of rain since then and we have gone back above it but given the dry conditions we expect the river to drop quite quickly back down to the level where we will have to institute conservation measures."

He said restrictions could be enacted by the end of the month if rain does not arrive in time.

"Lack of rain and low flows right at the start of summer would tend to suggest we are going to experience even lower flows than we have at the moment and flows that are likely to initiate calls for conservation."