A MAN in a gas mask allegedly sprayed terrified bank staff with an unknown substance during a botched hold-up in Frankston this morning.

Investigators have been told a man entered a Westpac bank on Wells Street shortly before 10.30am and allegedly sprayed two staff members with an unknown substance, now believed to be water, before demanding money.

He was arrested on nearby Young Street shortly after and is assisting police with their enquiries.

Shaken staff members have been treated at the scene but did not sustain serious injuries.

A woman, who works upstairs from the bank, said she was shocked to hear there was an attempted armed robbery.

Colleen Gardiner said she heard a package was thrown into the bank by the man.

“I thought it was pretty brazen considering it’s a fairly busy part of Frankson,” she told the Herald Sun.

Hayden, a caller to 3AW, said he’d just received a phone call from his wife, who was nearby the bank.

“She was walking through the car park and police came running saying, ‘get out of here, get out of here’,” he told Neil Mitchell.

“Apparently a fella with a gas mask and a gas canister has come out of one of the banks and police are roping off the area.”

The investigation is ongoing and people are being asked to avoid the area which is in lockdown.