MINYIP, Rupanyup, Donald and Wycheproof have drinking-quality water after the towns' water supplies were upgraded.
GWMWater started works in October 2013.
Floods in 2011 contaminated the water supplies.
GWMWater managing director Mark Williams said each town was now being supplied with drinking-quality water piped from a water treatment plant in a nearby town.
He said Donald residents would receive water from the St Arnaud Water Treatment Plant via a 39-kilometre pipeline.
Minyip's water comes from the Murtoa plant 24 kilometres away.
The plant will also supply water to Rupanyup.
Wycheproof residents' water is piped from the Charlton Water Treatment Plant via a 26-kilometre pipeline.
Yarriambiack Shire Mayor Andrew McLean said the project was a win for residents.
"It is a step forward as far as liveability in the area goes, having drinking water available from the tap rather than having to worry about buying bottled water or filtering water from the tank," he said.
"Our regulations around drinking water have gradually increased all the time. Where we used to be happy to drink from rainwater tanks, the water from them is probably seen as not as good as it used to be or as good as we thought it was.
"Having water from the Murtoa plant will alleviate any problems we have with quality."
Mr Williams said the decision to pipe water to the four towns used spare capacity in the existing water treatment plants.
"This provided significant cost savings compared with the traditional solution of building plants in each town," he said.
Mr Williams said the project provided drinking-quality water to more than 3000 people at a cost of about $12.5 million.