If your pigs are over six months old, you can't use lucerne. It's a legume, like peas, not a grass hay. And it's got way too much calcium in it for adult pigs -- you'd wind up with pigs with urinary tract stones.
I don't really know much about barley, but I've never heard of it being fed to guinea pigs. They actually need a grass hay -- that has the right about of protein that they need.
Grass can be used, but there are some pros and cons. You need clean grass that hasn't been exposed to pesticides or animal urine or feces. Once you start with it, you have to feed pretty much the same amount every day, because variations in the amount of grass can cause digestive upsets for guinea pigs. You also have to worry about the calcium content of the grass. Unless you have some way to determine that, you won't know whether you're giving them too much or not.
You're new to guinea pigs, and may not realize what a huge problem urinary tract stones are. They're incredibly painful for the pigs, and for your pocketbook. Most of us go to extreme lengths not to overload our pigs on calcium, although pigs can develop them even when eating a low calcium diet.
There are several Australians on the board -- maybe they'll weigh in on places to get hay for pigs.