Bus drivers were on the verge of walking off the job if something was not done about the spate of rock throwing at metropolitan buses, a transport workers union official warned yesterday.

"Without pre-empting anything, if someone gets badly hurt, I'd say we'll either pull the buses off the route or we'll pull the buses all together," union organiser Kevin Starr said.

Another bus was badly damaged on Whitfords Avenue on Saturday night when a champagne bottle was thrown through a window.

Shattered glass landed on seats but there were no passengers and the driver was not injured. It is the latest in a series of attacks in Perth that have injured bus drivers, passengers and car drivers.

Mr Starr said legislation should be strengthened to make it a penalty to throw a rock at a bus. Bus drivers should be included in mandatory sentencing legislation.

Attorney-General Christian Porter said last week he hoped to introduce a law that could see rock throwers jailed for up to five years.

Mr Porter said he was seeking legal advice about introducing a law similar to the one in NSW that made it illegal to throw rocks at vehicles or boats, even if the missile did not hit its target. Under the law, it was not a defence for the offender to claim they did not mean to hurt anyone.