check your states registration website
last year i bought secondhand 10 yr old bmw here in victoria, car was registered in NSW
now chap wanted to cancel his registration and get the remaining rego paid back to himself, that was ok by me
in short it is illegal to drive an unregistered car on the road, if you god forbid have crash you will not be covered for car or personal damage
i had to get temporary registration permit, you can buy for 1 to 4 weeks, this allowed me to drive car to get Road worthiness Certificate and get repairs only; you cant drive as per usual. this can be done(get temp permit) over phone and fax here in victoria, cost was about $80 for 3 weeks
stamp duty will have to be paid as well, paid when full registratio is obtained; this a tax on cars by state goverments! allup this can cost up to a grand(rego and stamp duty)
make sure you don't get ripped off, ?is car still under finance; this can be checked over the phone for a fee of about $30, get a written receipt for any money you give, keep details of seller and note down ID details(license and bills etc)
id car is still under finance by seller when you buy it and they do a runner and dont pay out loan then expect your car to be repossessed by finance company, you will have no come back, your money will be lost