How recently? This afternoon? last week? more?

(In the last hour we had what appears to be a DDOS attack on the network between 2.30 and 2.45, which would have caused slowdowns for people for that period - so if it's very recent, that would explain it)

If it's been happening for longer than the last hour:

Have you confirmed the following isn't responsible?

1) wifi congestion

2) other people in the house (or other devices) sucking up the bandwidth

(and sorry if that seems obvious - but you don't mention in the OP what you have checked yet, and those things are by far the most likely causes of slow broadband so should always be ruled out first)

Have you contacted our support team about it yet?

 ETA: as I type this, another one is happening. Network peeps are hustling to isolate it.

ETA2:  that second one has been stopped. Hofefully that is the last.

I don't have the full details at the moment - but looks like somebody got slammed in dunedin, which meant MOSTLY dunedin people impacted but also a few others around the country.