The FTB says the site was slow throughout Monday morning and into the afternoon. It then picked up mid-afternoon before struggling again around 5pm, when it was 65 percent over capacity. "It looks like we just had a lot more website traffic than we could handle," says the FTB's Denise Azimi.
She says 90 percent of the tax returns filed so far have been filed electronically - a slight increase from last year.
Azimi stresses that tax returns themselves aren't due until October. It's the deadline for payments that's been extended until Tuesday.
The deadline extension is for California tax payments only. Payments to the IRS are still due Monday.