On life support themselves, the Melbourne Rebels have driven another nail into the NSW Waratahs' coffin with a spirited 27-21 Super Rugby Pacific derby win in Sydney.

The cash-strapped Rebels, their very existence uncertain beyond 2024 as Rugby Australia mulls the side's future, rose from the dead on Good Friday to consolidate a top-eight spot with a crucial comeback victory at Allianz Stadium.

Tries in the last 10 minutes from inspirational Wallabies prop Taniela Tupou and winger Filipo Daugunu secured Melbourne a momentous win after the Waratahs looked to have pulled off a vital great escape.

"It's massive for us to get a win, after the last two weeks of not being able to get a win," said Rebels captain Rob Leota.

"To come away and win on the road is definitely big and gives us a lot of confidence going back home next week, so I'm pretty happy with that."

After three straight narrow defeats, the Waratahs were desperate to rebound and remain in finals contention.

Instead, coach Darren Coleman suffered a huge pre-game blow with powerhouse prop Angus Bell (back) joining fellow Wallabies forward Ned Hanigan (hamstring) and in-form flanker Charlie Gamble (calf) on the sidelines.

Coleman was forced to hand Australian under-20s front-rower Jack Barrett and Sione Misiloi unscheduled Super Rugby debuts.

The disruption may or may not have led to an error-riddled, though entertaining, opening quarter featuring frenetic handling and desperate, scrambling defence - but no points from either side.

Melbourne eventually opened the scoring in the 21st minute, somewhat against the run of play, with a try to winger Lachlan Anderson, set up from a quick tap and burst from halfback Ryan Louwrens inside his own territory.

The Waratahs levelled soon afterwards with a fine team effort when flanker Lachie Swinton finished a movement involving eight sets of hands and finally an improbable flick pass from prop Tom Ross to skip through and score under the posts.

But the Tahs suffered another major setback when lock Hugh Sinclair's clumsy attempted tackle on Anderson resulted in a head clash and a yellow card that helped the Rebels to a 10-7 halftime lead.

Melbourne might have lived to regret spurning multiple opportunities to extend their lead with penalty-goal attempts, instead opting for scrums and lineouts inside the Waratahs' territory early in the second half.

NSW produced one of the great blunders themselves, with replacement lock Miles Amatosero inexplicably failing to tap the ball from a penalty in front of the Melbourne posts with his team still three points down.

Coleman couldn't believe it, with the cameras capturing the coach pulling his hair out in the NSW box.

Langi Gleeson briefly eased the stress when the Waratahs' best performer charged over to give the home side a four-point lead on the hour mark.

But the Rebels' late double strike left the Waratahs languishing in 10th spot on the ladder with a treacherous run of matches to come.

They head to Canberra next week to play the Australian benchmark ACT Brumbies before facing a trio of tough New Zealand rivals in the Crusaders, Chiefs and Hurricanes.