Unable to locally verify the issuer's authority.

[robin]

In a terminal, when I run the following:
wget https://www.linz.govt.nz/sites/default/files/media/ntm/bsb/BSB_UPDATE.zip

I get an error message about certificates, as follows.
ERROR: cannot verify www.linz.govt.nz's certificate, issued by ‘CN=Thawte RSA CA 2018,OU=www.digicert.com,O=DigiCert Inc,C=US’:

The file is not downloaded. I contacted the maintainers of the site, they declared there is no problem with the certificate.

Of course, this probably isn't a problem with wget, or with curl which gives a similar error and the advice to go to this page:
https://curl.se/docs/sslcerts.html

Unfortunately that's way over my head, so I'm hoping someone helpful here can shed some light on the matter.

Thanks

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: wget 1.20.3-1ubuntu1
Uname: Linux 5.14.11-051411-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.20
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: GNOME-Flashback:GNOME
Date: Tue Oct 12 19:19:54 2021
InstallationDate: Installed on 2019-05-26 (869 days ago)
InstallationMedia: Ubuntu 18.04.2 LTS "Bionic Beaver" - Release amd64 (20190210)
SourcePackage: wget
UpgradeStatus: No upgrade log present (probably fresh install)