When I do pass my cap without topping up, my speed seems to drop to the point where pinging someone only has a 25% reply rate. It has been like this for over a year now.
Also you can just ring them and go through to the new customer signup option. Then just ask them to top you up. Rather than wait for 2 hours for the broadband helpdesk.
Slingshot also have a quirky thing happening with their bandwidth counter. It takes 3 days for any international traffic to be counted against my datacap. This is good in a way because I can usually get an extra 10gb topup credited if I call and complain that my internet has suddenly died within the first few days of the month. But still damn annoying.
I must admit that their customer service is [expletive removed] but pricing is really good.
Speeds seem to be good also when I do have data left: ~3mbps on the nzdsl speedtest and ~150kb for most torrents.
[Moderator edit (MF): expletive removed]