From the 70's/80's (80's more likely).

Set in Australia i think.

An experiment goes wrong killing everyone on the planet, except those people that died at the exact same time.

Anyone know it?

i think i do - but i have no idea what it was called - was there 3 of them at the end that survived?

yes, that's the one. god it's driving me mad!!

found it - its called

the quiet earth http://en.wikipedia.org/wiki/The_Quiet_Earth_%28film%29

its set in new zealand not australia.

been driving me mad since you mentioned it.

Good film