It is is expected residents of Strahan on Tasmania's west coast will need to boil their tap water for drinking for at least another week after the supply became contaminated with E. coli.

More than 500 homes are affected, after samples taken during routine testing on Tuesday were found to be contaminated with the bacteria.

TasWater crews have arrived in the town to try to work out the cause.

Lance Stapleton from TasWater told 936 ABC Hobart it would take time to inspect all the infrastructure.

"We're working methodically through the system and we're trying to identify the cause of contamination so that we can rectify it as quickly as possible," he said.

"I think it's going to take at least a week, just by the time we actually go through the system and work out what's wrong, do some retesting and get several clear tests, it'll be at least that long, depending on the nature of the problem it could be a bit longer."

The town receives water from a treatment plant and several reservoirs.

West Coast Mayor Phil Vickers said he hoped TasWater could find the cause quickly.

"It's a necessary inconvenience, it really is, I suppose in days gone by before TasWater one bad sample out of five might not have caused such an alert but you can't be too careful these days," he said.

Water for drinking, brushing teeth and cooking needs to be boiled before use.