Wellington City Council says problems with a sewage leak in Johnsonville appear to be long standing.
A resident complained that untreated sewage was running onto the Johnsonville off-ramp on Saturday and cars were driving through it.
A council spokesperson said there have been issues for the past couple of weeks but workmen are trying to fix the problem and establish if it is being caused by a system blockage.
He said only a small amount is running onto the motorway and it is not a health hazard.
But Jo Ralph, who lives above the motorway, said there is a bad smell and she can see sludge and toilet paper on the bank behind her property.
