Builders are sounding the alarm on a union push to restrict labour hire and shut down construction projects every second Monday, warning it will increase costs by up to 40 per cent and directly reduce productivity.

The Electrical Trades Union (ETU), representing one of the industry’s most critical trades, has been negotiating a new agreement with major contractors in NSW that lifts pay by up to 7 per cent a year, restricts suppliers to union-approved firms and imposes fixed rostered-days-off (RDO) that would apply a nine-day fortnight mid-project.