I am not an expert, but from my reading on the subject, there is a whole range of frost tolerance in Grevilleas- a lot depends on where they were bred. Perhaps you should consult plant lists from some of the nurseries and/or SGAP (Soc. for Growing Aust. Plants). There are Grevilleas that are adapted to the ACT/Monaro area, whilst others are from the tropics! Don't take as 'gospel truth' all you read on the printed labels. Do a little bit of independent research as well before you go ahead and purchase.

Here is a link that might be useful: Society for growing Australian plants

Were they planted where they received the morning sun? The sun on frosty leaves can often cause a sudden death on a plant which may otherwise survive.

thanks wattleblossum that is probabaly the reason as that is the only sun they would recieve but it doesnt explain why the rest of my yard hasnt died off

Sorry. The other plants in your garden are probably more frost hardy and can withstand the sun shinning on them before the frost has melted. Often a plant with boderline hardiness can still be grown if it can be positioned where it doesn't get the sun until a bit later in the day. Years ago, you'd often see people outside, in their pyjamas, hosing the frost off plants before the sun reached them, but of course those days are now gone.
You might be saying that you have several of the same type of grevillea and are asking why you've lost some but not others? Well, I've found that even a few metres can be the difference between death and survival, especially if your garden is on a slope. Also nearness to a fence, a building, or some other structure can make a difference.
I'm afraid there is always the possibility that you will lose more of the same grevillea as winter progresses.
Sometimes plants just burn from frost and don't actually die. In this case it is important not to remove the burnt bits as they protect the rest of the plant. So wait until after the last frost before tidying up.
Often a plant which is susceptible to frost when young will become more hardy as it matures.
All very complicated I know, but I hope it's been of some help.