The police chief and mayor of Venice have pledged action after four Japanese tourists said they were presented with a bill of €1,100 (£970) for four steaks and a plate of mixed grilled fish, washed down with water.

Another three women in the same group suspected they might be taken for a ride at the restaurant near St Mark’s Square and ate elsewhere, the Ansa news agency reported, but still ended up paying €350 (£308) for three dishes of seafood pasta.

“We will thoroughly examine this episode over the coming days. We’ll make sure the complaint was justified,” the mayor, Luigi Brugnaro, said on Twitter. “If this shameful episode is confirmed, we’ll do all we can to punish those responsible. We are for justice, always!”

It is unclear why the tourists, among an estimated 30 million who visit Venice each year, did not refuse to pay the bill. They filed a complaint after leaving the city in Italy’s north-east and upon returning to Bologna, where they were on a study visit.

A spokesman for the restaurant, which according to local media is owned by Chinese investors and managed by an Egyptian, reportedly told journalists he had “no recollection of any problems with Japanese customers”.

A residents’ action group, Gruppo 25 Aprile, jumped on the case as further evidence of the risk that tourists run of being ripped off. “We defend local residents, and whoever puts the good name of Venice at risk harms all Venetians,” it said.

The group’s spokesman, Marco Gasparinetti, said that in the immediate vicinity of St Mark’s Square in the historic city centre, only 1% of restaurants were still owned and operated by locals.

With Carnival fast approaching, he said, the group planned to publish “a user’s guide for visitors on how to survive in Venice, with details on the kind of situations to avoid” on its Facebook page. Other associations are considering introducing a seal of approval for strictly local businesses.

In November, a British family of three from Birmingham were charged €526 (£463) for lunch – including €297 for a platter of grilled fish – in the Trattoria Casanova on a busy tourist route between St Mark’s Square and Santa Maria Formosa.