A fire which burned through 100ha of hillside near Arthur's Pass last week and four more fires on Saturday are suspected to be the work of an arsonist.

Now firefighters are hoping they don't strike again this weekend.

Like the first fire, which broke out on Monday last week and took more than two days to put out, the latest fires were in the area of Mt Horrible. They were spotted by a passing train driver just after midnight.

They ranged in size from 50m by 20m up to more than a hectare.

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Department of Conservation deputy rural fire officer Bruce Janes said a rural fire investigator had determined the fires had been deliberately lit, and arson was suspected.

"There are a lot of tired firefighters and now some upset firefighters [that have had to] keep on going back there," Mr James said.

"It's a huge burden on the taxpayer fighting these fires - lots of helicopters, lots of personnel, it's quite expensive."

The first fire had cost in excess of $150,000 and while the bill hadn't come in for the latest four yet, it would be in the tens of thousands.

"We're hoping he doesn't strike this Friday night, Saturday morning," Mr James said.