It does look undervalued when looking at price v book value....

It does look undervalued when looking at price v book value. Book value is also a function of income that can be generated from each property.
In my opinion it is too bearish to think the book value would be slashed in half (which would bring it down to the roughly equal the SP which isn't neccessarily expensive even at that point)
A big question which the market is concerned about is the gearing and what effect a significant write down of assets has on their debt covenants.
A capital raising is still a risk and it would be especially dilutive at these SP levels.

Another risk is that some of the anchor tenants are in trouble.. i.e. myer &DJs.. so how much value is lost if they disappear?

Overall in my view, the stock has been beaten down with the widely adopted thematic that physical retail is becoming redundant - which I don't buy (may change to being more showroom style - but physical front creates marketing value and gives the brand clout and visibility whilst capturing people that may not usually come accross it).
Never the less over 40% of scentre's tenants now are 'consume on site' places.. eg. Restaurants, gyms, health, entertainment - that will resume as normal with re-opening.
And ultimately, if the centres continue to generate foot traffic, then the tenancies will remain attractive to businesses looking to capture it.

In relation to foot traffic - with more people working from home, there is more of a desire to get out of the house and westfield centres are an attractive option for this. We have seen that play out with crowds in the centres over the last few weeks.
(Although may be hampered by going back into lockdown in the short term, and will remain a volatile 'risk' stock during this period)