I'm in Perth. I'd like to go SUPing on the Swan River in the city but only if it's calm and smooth. What is the max Wave Height and Wind Speed I should look out for as a good indicator for these conditions? Thanks.

Waves irrelevant thats the ocean
Wind under 8knots - careful as the media often confuse kph and kn.

8 knots.

Eg on the graph right now, wednesday midday, saturday morning. Only if you look soon cos now I've jinxed it

Depends where you want to go SUPing
Fremantle to Point Walter is good place in the morning.
Melville east and West - Anywhere along the south banks will be calm on the river in a SE/SW wind. After 1pm the seabreeze will make a wind swell worth rolling along with up Melville waters from Pt Walter to Pelican point.
Perth waters - In front of the city between the Narrows and Causeway bridges, again, the south shore line is usually the calmest. If there is a strong easterly, this can area can get quite choppy.
Up river from the Causeway is fairly calm on most breeze strength and direction.

Forgot to mention too - start off paddling into the wind.