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 on March 21.
The hired helicopter was fitted with older-style aluminium fuel tanks known to pose a fire hazard.
The maker, Robinson, has recommended R44s be refitted with flexible, bladder-type fuel tanks to reduce the risk of post-accident fires.
A preliminary report on the Bulli Tops crash by the AustralianTransport Safety Bureau (ATSB) this week found there had been a post-accident fire.
That report prompted the Civil Aviation Safety Authority (CASA)to strengthen its stance and warn R44 operators their machines would be grounded if the new tanks were not fitted by the end of April.
The ATSB report said R44 accidents resulted in a significantly higher proportion of post-impact fires than for similar helicopter types.
"The ATSB remains concerned at the significant risk facing many R44 helicopters and has recommended that CASA take further action to ensure compliance with the manufacturers service bulletin," it said.
CASA spokesman Peter Gibson said operators had known since December 2010 that the fuel tanks needed to be replaced and they had known since September last year that the deadline was April 30.
"What we've said to them now in the wake of this latest accident is ... you must achieve it by the 30th of April or you won't be able to fly your helicopter," Mr Gibson told AAP.
He said about a quarter of the R44 fleet, around 100 aircraft,had already been refitted with the new tanks and CASA was confident that another 200 would have been refitted by the end of the month.
That left about 100 helicopters that CASA wasn't sure would make the deadline and could be grounded.
