Ok so it looks like Telstra have restricted IG site access through their DNS as part of protecting people from bad sites which I suspect IG is trying to sort out ASAP as their use is legitimate. Telstra support were far from helpful and even said I needed to pay for their platinum service. When I suggested I would report this requirement to the Ombudsman they gave me a one off access but the Platinum team had no knowledge of the issue or how to resolve beyond trying to get me to fill in a security complaint form!
So here was my workaround for a windows 10 PC (no promises it would work for others).
I went to my computer settings (typing in 'settings' using the magnifying glass in bottom left corner of my computer)
I then Selected 'Network & Internet' then WiFi properties
Scrolled towards the bottom of the page for 'IP settings'
Took a note of my IPv4 address number and IPv4 gateway number.
Pressed the button to edit the 'IP assignment' and selected 'Manual'
Made IPv4 ON - then
IPv4 address: added my IPv4 address number I took down before
IPv4 Subnetprefex length: 24
IPv4 gateway: added my IPv4 gateway number I took down before
Preferred DNS: 1.1.1.1
Alternate DNS: 8.8.8.8
Pressed Save
As a final step I cleared the DNS cache through the Run Command prompt by searching for 'Command or CMD' using the magnifying glass in bottom left corner of my computer. This brought up a black screen with C:\users\###> prompt - the ### was my user name (different for each user)
I then typed in after the C:\users\###> the following
ipconfig /flushdns
(note the space between ipconfig and /flushdns is needed)
I also cleared my cache on my Chrome Browser (three dots) top right and selected Settings/Privacy and Security/Clear Browsing data. Then went into IG and logged in successfully.