it's not work related so it would be perfectly legal to block it.
...but I doubt that there is any site that is blocked from govt offices, that is more like a private sector trait.
For people in government who work with policy, media liaison, safety & security, rapid response, media monitoring, public education etc it is work related to know whats going on with various political parties. Imagine a speechwriter / spokesperson who is supposed to write a 50 page Parliamentary rebuttal or respond to allegations made in the media or by other parties. How are they supposed to research their stuff if they don't have access to the original documents/statements made by those opposition parties?
If they block one party's site, they should block them all. Otherwise it becomes like those cases where people wearing COPE shirts can't get social grants.
Quite a few government depts block youtube, facebook (the memo was hilarious, but valid) & most sites with streaming audio or video. This girl I know had to write a series of sarcastic and betchy emails to the IT office to get access to Radio 702's website. Her department's minister was interviewed on the station, she'd gone to a lot of trouble to purchase advert airtime on 702 and they also had a lot of reports about the department on their site. Callers also phone in and ask to speak to officials from there, make complaints etc.
She had to keep track of those things, make recordings, write media statements and responses, print the stories from 702's news reports & forward enquiries and complaints made on air, to the relevant people in the dept. But she couldn't open the site because they have streaming audio. She couldn't do her job properly because of that.

It took about 5 emails and 3 phonecalls to get the point across, that no, she didn't want to d/l music so she can booty dance in the office

, she actually needed to view that site for work. Thankfully, 702 now has a separate site with just news stories in text.