All Robinson R44 helicopters without upgraded fuel tanks will be grounded from Wednesday under orders from the Civil Aviation Safety Authority (CASA).

The authority says R44 operators were first told they needed to make sure their helicopters were fitted with flexible fuel tanks in December 2010.

The safety measure is needed to reduce the risk of fire after an accident.

Four retired CSIRO scientists were killed when the R44 helicopter they were in clipped a tree and burst into flames when it crashed at Bulli Tops in NSW in March.

The hired helicopter was fitted with older-style aluminium fuel tanks known to pose a fire hazard.

CASA says more than 170 R44 operators have not complied with a direction to notify the safety watchdog of their helicopter's status and maintenance program.

These reporting failures prompted the order for grounding from May 1.