DVD

If you don't live in Australia/NZ and have a Region Free DVD Player, the Australian releases are the better deal by far:
The N.American Region 1 releases cost more than twice as much per season.
And you can't even get seasons 3 and 4.
I don't know about region 2 availability, but I do know that season 3 and 4 have not been released in English in region 2 (Only in Dutch as far as I know)

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Region 1 NTSC (North America)
Seasons 1 & 2 can be found at CD Universe
Each part is around $20US

Season 1 (1998)
Part 1:
http://www.cduniverse.com/productinfo.asp?pid=2246259
Part:2:
http://www.cduniverse.com/productinfo.asp?pid=2246277

Season 2 (1999)
Part 1:
http://www.cduniverse.com/productinfo.asp?pid=6775004
Part 2:
http://www.cduniverse.com/productinfo.asp?pid=6774971&style=movie

Seasons 3 and 4 haven't ever been released as region 1 or 2

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Region 4 PAL (Australia)
Sols as 2 season collections only
Sells for around $40 Australian

Seasons 1 and 2:
http://www.ezydvd.com.au/item.zml/792670

Seasons 3 and 4:
http://www.ezydvd.com.au/item.zml/792671

They are frequently out of stock though.

You might have better luck at shop.abc.net.au, as they broadcast the series in Oz.

Seasons 1 and 2:
http://shop.abc.net.au/browse/product.asp?productid=741972

Seasons 3 and 4:
http://shop.abc.net.au/browse/product.asp?productid=741980

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