International roaming vs. buying a local SIM card

Right, I did a search but didn't really find quite the right topic (but am sure a Mod or you lot will point one out).
Basically, it's a 2 month stint in NZ & Australia, I have a O2 Genion Germany contract. As I will be travelling alone for much of the time, yes I do want to be contactable whether within those two countries locally or from Europe (my parents use a call plan for very cheap calls to NZ/Aus inc. to mobiles and being contactable by them is my main issue) and am unlikely to use call shops (more likely to use Skype/VOIP for longer calls) and do love a text message or several.
Is buying a local prepaid SIM a good idea?
And the Vodafone Australia site appears to have words but not in a form I can understand, so hopefully someone here can help. Thanks.

I'd recommend just buying a prepaid chip for your current handy, be way cheaper. NZ & Aus Vodafone have reciprocal agreements in terms of their tarifs so you will pay rates as per your plan in either country, ie no roaming charges.

Sweet - because that Vodafone site made no sense whatsoever and I was feeling a bit daft. Ta.

try the Vodafone NZ site, we're better at the whole speaking/writing English gig than those Aussies

It's obviously a lot cheaper to make calls (and receive them) if you buy a prepaid card over there. The only trouble is that you'll have to give this number to all the people you expect would be ringing you. Roaming charges for O2 in Australia would be expensive.

To cover both bases, you can get deals in Australia where they give you a phone and $20 worth of credit for like $30. Then you can carry two phones around with you. They do it everywhere, just go to a shopping mall and they'll be able 100 mobile phone shops in there. Then see what they have to offer.