I've developed several infographics (below) comparing the open data performance of nations, looking at which have national open data sites, how many sites they have across different government levels and how many datasets have been released through their national sites.
It's not a way to judge 'winners' and 'losers' - or even to compare the relative performance of countries. However it provides useful information on who is doing what and how deeply open government has been embedded in the thinking of agencies. This said...

There are 41 countries listed (by data.gov) as having open data websites, out of almost 200 nations.

In their national open data sites, in total, these nations have released at least 1,068,164 data sets (I was unable to get a count from China, Timor-Leste, Tunisia or Sweden's national open data sites), for an average of 28,869 and a median of only 483 - due to a few high release countries (US, France, Canada).

How do Australia and New Zealand rank?
As people will look for this anyway, based on the number of datasets released as of January 2013, New Zealand is 9th (with 2,265 datasets) and Australia 11th (with 1,124 datasets).

Between us is Estonia, with 1,655 datasets.

The top nations above New Zealand are, in order: US (378,529), France (353,226), Canada (273,052), Denmark (23,361), United Kingdom (8,957), Singapore (7,754), South Korea (6,460), Netherlands (5,193).

Infographics





Raw data
The raw data is available in a spreadsheet at: https://docs.google.com/spreadsheet/pub?key=0Ap1exl80wB8OdFNvR3dja3E4UGtVVi1LMU11OFBmR1E&output=html

Caveats
When it comes to nations and states there's few absolute measures, there's simply relative performance - across jurisdictions or across time.

These comparisons are often flawed due to variations in data collection, lack of information or differences in approach, however there can still be value in 'placing' nations, identifying opportunities, challenges, flaws and risks.

My work above is not a measure of the success of open data itself, but provides a relative indicator of which governments have been more successful in embedded open government principles in agencies, and how deeply. It also provides insight into wich nations are working in this space.

My data spreadsheet is also a useful 'point in time' reference to track changes over time.

Note that I was unable to count open data released outside of national open data sites - there's a lot more of this, however it can be harder to locate. Due to the sheer number of state-based open data sites (210), I've not yet done a tally of the datasets they've released, only of the 41 national sites. Watch this space :)

The data may not be 100% accurate due to differences in the approach to releasing data. data.gov provided the list of data sites and I drew specific information on datasets and apps from all 41 national open data sites, each with a different design and functionality and across over a dozen languages.

Please let me know of any inaccuracies and I will endeavour to correct them.