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Hi jexy1!

Welcome to TT!

Hey a friend and I (both young females 21yo) are looking at doing some volunteering in both Zimbabwe and Namibia

You'll probably need a specific visa for volunteer so I would get in touch with your nearest embassy/consulate of Zimbabwe and of Namibia in order to know about it. If you already know with what organization you are going to volunteer, you could ask them too as they should have experience with previous volunteers.
It seems Namibia doesn't have an embassy in Australia but Zimbabwe has one.

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It's a long but nonetheless pretty direct route to get from Australia to Zimbabwe so I'm not sure what kind of "easier" suggestions you have in mind... nonetheless rather than taking the bus all the way to Zambia I'd suggest you look into flying to Livingstone. There are a few low cost carriers operating in South Afirca, both 1time (https://www.1time.aero/) and Kulula (http://www.kulula.com/) fly to Livingstone. They also appear to have recently started selling flights to Vic falls as a codeshare with British Airways, although my experience is it's usually cheaper to fly into Zambia than Zimbabwe (however you'll need to keep in mind the additional visa costs in doing so...).

Zim visas for Australians are $30US for single entry and $45 for double. Australians can obtain these on arrival. If yo're flying into Zambia you'll have a $50US visa to pay for - this applies even if you're just transiting through Zambia to get to Zimbabwe. A few years back you could definitely enter Namibia as a tourist on arrival with no fee applicable, however you may wish to seek more up to date info to ensure things haven't changed. I'm not sure what the visa situation is if you're intending to volunteer, hopefully someone else can jump in on that point.