The water is now being pumped away from the area and various drains cleared to assist with this. No one has been evacuated.
In Invercargill, properties on Bainfield and Retreat Rds and Venus St have also been affected by flooding.
Residents of the Venus St property have been offered alternate accommodation.
People are advised to avoid travel in Southland if possible, due to flooding affecting a number of roads.
McKay said contractorswere signing as many roads as possible but were stretched in getting everywhere and in having enough signs.
The heavy rain has also caused slips also in Stewart Island and the Catlins.
Emergency Management Southland has opened its emergency coordination centre in response to the flooding.
McKay said he urged Invercargill and Southland residents to avoid non-essential travel, including in the city, where water could be pushed into low-lying properties by cars driving through ankle-deep water in some places.
Southern District Police said there was "significant flooding" in Riverton and a number of streets had been blocked off.
People were asked to avoid Taramea Bay and Walker Street, "as walking down these roads pushes water into the houses".
Colac Bay and Orepuki should also be avoided, police said.
The coordination centre had been set up this morning to gain a clear assessment of the situation both in the city and Riverton and other rural areas, McKay said.
Invercargill firefighters were helping to pump out some flooded properties, including in the Bainfield Rd area.
However, "there's so much water about at the moment and it flows through" and it was difficult to pump under those circumstances, he said.
An eyewitness said that ankle-deep flooding had affected part of the central city and some "patches" elsewhere in the city.
McKay said this flooding was "completely different" from earlier flooding in February, which had not affected the city itself, and which was river-related flooding.
"It's just the amount of rain we've had," he said.
The latest flooding had resulted from continuing heavy rainfall, and the city stormwater system was finding it hard to cope.
A key message was to ring 111 for emergencies, but, generally, local councils should be contacted about flooded roads, he said.
"Avoid travel if it's not needed and sit in front of a nice warm fire," he advised.
- additional reporting RNZ